System and method for positioning using hybrid spectral compression and cross correlation signal processing

ABSTRACT

The present invention relates to a system and method for positioning and navigation using hybrid spectral compression and cross correlation signal processing of signals of opportunity, which may include Global Navigation Satellite System (GNSS) as well as other wideband energy emissions in GNSS obstructed environments. Examples of these signals of opportunity include but are not limited to GPS, GLONASS, cellular Code Division Multiple Access (CDMA) communications signals, and 802.11 Wi-Fi. Combining spectral compression with spread spectrum cross correlation enables extraction of code and carrier observables without the need to implement the tracking loops (e.g. Costas tracking loop) commonly used in conventional GNSS receivers. For applications where dynamics and transmission medium may make it difficult to continuously track carrier phase, the hybrid approach of the present invention has significant utility.

RELATED APPLICATIONS

This application is a continuation of pending U.S. patent applicationSer. No. 13/356,281, filed 23 Jan. 2012, which is a continuation-in-partof pending U.S. patent application Ser. No. 13/269,426, filed 7 Oct.2011, which is a continuation-in-part of abandoned U.S. patentapplication Ser. No. 13/029,966, filed 17 Feb. 2011, which claimspriority from U.S. Provisional Application No. 61/391,517, filed 8 Oct.2010; and, which is a continuation of U.S. patent application Ser. No.12/372,235, filed 17 Feb. 2009, now U.S. Pat. No. 7,916,074, which is acontinuation of U.S. patent application Ser. No. 11/697,575, filed 6Apr. 2007, now U.S. Pat. No. 7,511,662, which claims priority to U.S.Provisional Application No. 60/745,928, filed 28 Apr. 2006, all of whichare hereby incorporated by reference in their entirety as set forthherein.

FIELD OF THE INVENTION

The invention generally relates to a system and method for positioningand navigation of assets and, more particularly, to a system and methodfor using hybrid spectral compression and cross correlation signalprocessing using signals of opportunity.

BACKGROUND OF THE INVENTION

The global positioning system (GPS) has fundamentally changed themethods of navigation, location tracking, and time synchronizationworldwide. With thirty-two satellites on orbit, the GPS providescontinuous positioning service at almost anyplace signals can bereceived. With the advent of low-cost positioning sensors using GPS,accurate to a few meters, there has been a proliferation of thetechnology into core infrastructures including power systems,communications, transportation, and military. The importance of thiscapability as a national asset cannot be overstated and is highlightedby the fact that many other nations are now either operating ordeveloping their own GNSS, including Russia, Japan, China and theEuropean Union.

Despite its many advantages, GNSS has one significant drawback:satellite-based navigation systems signals are typically very weak asthey reach the positioning receiver. In some cases, like the GPS, thisis a key part of its design, but practically it is difficult to operatehigh power transmitters on orbit. These weak signals make it difficultto operate positioning receivers in obstructed environments, such asindoors, as the obstructions will tend to attenuate the signal power andrender it useless for positioning or, at the very least, substantiallydegrade the overall measurement capability.

While significant effort has been made to overcome these limitations,particularly Assisted GPS and High-Sensitivity GPS, in practical termsmeter level positioning in obstructed environments using GNSS is notfeasible for broad usage. To provide positioning in obstructedenvironment another class of positioning technologies has been developedknown as real time locating systems (RTLS), which derive from radiofrequency identification (RFID) technologies.

Using a variety of ranging methods, such as time difference of arrival(TDOA), Received Signal Strength (RSS), fixed reader, and landmarktagging, RTLS offers a variety of positioning capabilities andaccuracies. The most advanced and versatile systems tend to use TDOA andcan offer positioning accuracy to within a few meters. Some of thesystems even claim sub-meter accuracy, though this tends to be in highlycontrolled environments.

While promising, RTLS systems are very expensive to install and operate.When high accuracy is needed, the cost and complexity of the equipmentcan make it all but impractical except for a few limited applications.RTLS offers a variety of solutions that can be tailored to fit a varietyof applications; however, when compared to the relative simplicity andwide availability of GNSS based positioning they all are less thandesirable.

Further, for combined applications requiring positioning in both localarea obstructed and wide area unobstructed environments, options areextremely limited as neither GNSS nor RTLS can satisfy the requirementalone. Combined RTLS and GNSS systems are impractical due to the factthat they are largely incompatible and are difficult to integrate and,as a result, very expensive. Several attempts have been made to adaptcommodity GPS receiver technologies using pseudolites to provide RTLScapabilities. While attractive in concept, these solutions are at besttoo expensive and power intensive to be practical in addressing many ofthe RTLS applications and at worst they are illegal to operate in muchof the world as they tend to jam normal GPS operations.

Accordingly, there is a need for a cost effective, highly accuratepositioning technology that operates equally well in obstructedenvironments using locally deployed beacon reference points and canutilize GNSS reference points such as a GPS satellite for wide areaunobstructed environments.

SUMMARY OF TERMS

The following definitions of certain terms are useful to provide afoundation for the discussion of the preferred and alternativeembodiments of the present invention.

“Almanac” means information describing the configuration, currentphysical state, or predicted future physical state of a reference pointor physical state sensor. This information may be internally generatedby a reference network processor or be provided by an external source(e.g. GPS receiver for GPS almanac and precision ephemeris). Typicallyalmanac information has a time of applicability and is stored in aformat that makes it relatively easy to use for physical stateestimation.

“Almanac correction” means corrections to almanac information. Thesecorrections are typically adjustments to one or more elements of analmanac and are more compact in size when compared to a full almanacrecord thus reducing bandwidth and storage requirements.

“Configuration data” means information that defines the systemconfiguration and relationship to external references. Configurationdata includes specifications of reference points, coordinate systemtransformations, and external time transformation data. The systeminformation may also include security attributes, physical state sensorregistrations and specifications of integrity performance criteria.

“Coordinate system fiducial reference” means a known or acceptedlocation in the coordinate system frame of reference that is determinedto accuracy better than the accuracy of the system end-user performancerequirement.

“Differential observables” means the observables that are formedwhenever observables from two or more interceptors are differencedproducing a differential measurement that effectively cancels thesystematic errors due to the uncertainties in the physical state of anemitter. Note that there are 1st, 2nd, and higher differencedobservables. The preferred embodiment typically uses first differences.

“Emitter” means any object that produces an energy emission.

“Energy emission” means structured or unstructured energy propagated insome transmission medium that can be intercepted and processed.Structured emissions include any emissions whose characteristics areknown and are deterministic and predictable in some manner. Unstructuredemissions are anything that are not considered structured and typicallyhave random characteristics.

“Interceptor” means any object capable of intercepting at least oneenergy emission.

“Location sensor” means a physical state sensor configured to produceobservables useful to the determination of position.

“Navigation processor” means a physical state estimator configured toprocess observables for at least one physical state sensor resulting inan estimate of the physical state of the physical state sensor. Physicalstate estimation can be implemented by any number of means. Thepreferred embodiment uses a combination of stochastic estimation methodsincluding least squares, Kalman filtering, and hybrid methods.

“Observable” means a measurement of the intercepted energy propagated insome transmission medium between emitters and interceptors.

“Physical state” means the physical characteristics relative to areference frame of a device comprised of at least one or more of thefollowing: position, attitude, clock and temporal derivatives. Positionand attitude may be in one, two, or three dimensions. Position is ameasurement of linear distance along one or more axes. Attitude is ameasurement of an angular rotation about some axis. Clock is themeasurement of time. Temporal derivatives are the time derivatives ofthe primary physical characteristics.

“Physical state estimate” or “PSE” means a computed estimate of physicalstate derived from observables.

“Physical state estimator” means a system element that processesobservables given previously defined configuration data producing aphysical state estimate.

“Physical state sensor” means a system element that is used to sense thephysical state. The physical state sensor may be an energy interceptoror an emitter depending upon the configuration.

“Reference point” means a system element acting as a point of referencefor measuring position of one or more location sensor(s). A referencepoint element can be either an emitter or a receiver of energypropagated in some transmission medium. They can be placed at knownfiducial points within the coordinate system reference frame. Referencepoints can also be moving, or of external origin such as quasars,satellite signals of opportunity, and any other emitter of energy. Theprimary characteristic of reference point is that one or more physicalcharacteristics are known prior to estimation of the relative physicalstate between the reference point and a physical state sensor.

“Ranging signal” means a structured energy emission purposefullydesigned to have appropriate characteristics to be useful in measuringthe range between an emitter and an interceptor.

“Ranging signal transmitter” or “RST” means an emitter that transmits aranging signal. This can be a global navigation satellite, a localbeacon, or any transmitter that produces a signal that can be exploitedas a ranging signal.

“Reference network processor” means a physical state estimatorconfigured to estimate the physical state for at least one referencepoint with respect to a second reference point and subsequently usingthe resulting physical state information to update almanac andcorrections information and other related configuration data for thesystem

“Reference SCT” means a spectral compressor and translator that isdesignated as a reference point in the system.

“Spectral compressor and translator” or “SCT” means a physical statesensor configured as an interceptor that processes intercepted energyemissions using at least one method of spectral compression producingobservables that can be used for physical state estimation.

“Spectral compression” means a process of extracting the changingphysical characteristics in the form of amplitude, phase and temporalderivatives of the intercepted energy as it propagates through atransmission medium without regard to the preservation of informationcontent potentially modulated within the energy emissions. The processof extraction utilizes at least one or more known physicalcharacteristics of the energy emission and emitter to distill widebandspectral content into a narrowband regime, which preserves the physicalcharacteristics. The distillation of wideband spectral content can beperformed without regard to modulated information content, enablingeffective process gain that yields high signal to noise ratio forextraction of the physical characteristics.

“System controller” means a system element (typically software) that hasthe responsibility to coordinate system operations managingconfiguration, calibration, and coordinating the flow of information toother elements in the system. The system controller implements timingand control functions needed to coordinate other system functions toprovide a certain performance and quality of service. Note thesefunctions may be physically implemented in a single controller ordistributed/shared amongst a group of controllers depending on specificimplementation requirements.

“Time reference” means an external signal that provides external timeand frequency information that is useful for synchronizing the system'stime and frequency reference. One of the most common external timereferences is universal time coordinated (UTC) and GPS time, enablingthe system time and frequency references to be linked to those specifiedsystems.

“Transmission medium” means any medium capable of propagating energy insome form; mediums include free space, liquids, solids and gases.

SUMMARY OF THE INVENTION

The present invention provides a system and method for determining thephysical state and principal position of a physical state sensorrelative to known reference points that may include both globalnavigation satellites (e.g. global positioning system (GPS)) and localbeacons such that proper coverage is provided even when the globalnavigation satellite system (GNSS) is not available or otherwiseobstructed. The invention presents a system and method for abeacon-based local area location system utilizing RF (or other signals)to provide ranging signals to one or more location sensors.

An exemplar embodiment of the system of the present invention forproviding physical state information within a configured environmentincludes at least one emitter that emits energy within a transmissionmedium; at least one interceptor that receives energy propagated througha transmission medium from the emitter, wherein the interceptor isconfigured to process the received emissions using spectral compressionto produce a set of observables suitable for physical state estimation.The system communicates the set of observables to a physical stateestimator, which is configured to determine a member of the relativephysical state between the interceptor and emitter based on the set ofobservables received from the interceptor. The system then reportsdetermined member of the relative physical state based on the set ofobservables received from the interceptor.

An exemplar embodiment of the method of the present invention forproviding physical state information within a configured environmentincludes the steps of emitting energy from at least one emitter througha propagation medium; intercepting the energy emission at theinterceptor; processing the received energy emission using spectralcompression to produce a set of observables associated with theemission; communicating the set of observables to a physical stateestimator; receiving configuration data pertaining to the deployment andconfiguration of the emitter and interceptor within the configuredenvironment; determining a member of the relative physical state betweenthe interceptor and emitter based on the set of observables and theconfiguration data; and reporting the member of the relative physicalstate.

The resulting alternative embodiments of the present invention overcomethe disadvantages associated with current systems and methods andprovide a cost effective, simple to implement and rapidly deployablesystem with a complete stand alone method for physical state estimationusing either local area beacons and/or wide area GNSS satellites such asGPS.

BRIEF DESCRIPTION OF DRAWINGS

Preferred and alternative embodiments of the present invention aredescribed in detail below with reference to the following drawings.

FIG. 1 is a logical systems diagram showing the components of theinvention including ranging signal transmitters, spectral compressor andtranslators and the processing components to determine the physicalstate using intercepted energy in accordance with an embodiment of thepresent invention.

FIG. 2A shows the integration of the invention with existingcommunication assets in accordance with an embodiment of the presentinvention.

FIG. 2B illustrates the components of the spectral compressor andtranslator integrated with the physical state determination processor inaccordance with an embodiment of the present invention.

FIGS. 2C and 2D illustrate the block level components of the spectralcompressor and translator integrated with communication assets inaccordance with an embodiment of the present invention.

FIGS. 2E and 2F show additional block level integration scenarios of thespectral compressor and translator in accordance with an embodiment ofthe present invention.

FIG. 3 shows a logical diagram for a scenario in which the invention iscombined in a hybrid operation mode with GNSS signals in accordance withan embodiment of the present invention.

FIG. 4A illustrates the detail of the ranging signal transmitter inaccordance with an embodiment of the present invention.

FIG. 4B illustrates the generation of the ranging signal within the RSTin accordance with an embodiment of the present invention.

FIG. 5A illustrates the functionality of a spectral compressor andtranslator in accordance with an embodiment of the present invention.

FIG. 5B illustrates the functionality of the channel processor componentof the SCT in accordance with an embodiment of the present invention.

FIG. 6 illustrates a physical state estimator, which converts observeddata to physical state elements in accordance with an embodiment of thepresent invention.

FIG. 7 shows the process of federated filtering to generate a referencecorrection data set in accordance with an embodiment of the presentinvention.

FIGS. 8A and 8B illustrate the difference between differential relativeand absolute positioning in accordance with an embodiment of the presentinvention.

FIG. 9 illustrates a 3D positioning deployment scenario in accordancewith an embodiment of the present invention.

FIG. 10 illustrates a deployment scenario in which both RST and GNSSsignals are available for hybrid positioning in accordance with anembodiment of the present invention.

FIG. 11 illustrates an application of the invention for search andrescue operations in accordance with an embodiment of the presentinvention.

FIG. 12 is a logical systems diagram showing the components of theinvention including an emitter, interceptor and physical state estimatorused to determine physical states using intercepted energy in accordancewith an embodiment of the present invention.

FIG. 13A illustrates a method for providing physical state informationwithin a configured environment in accordance with an embodiment of thepresent invention.

FIG. 13B illustrates the energy emission interception and processingmethodology for providing physical state information within a configuredenvironment in accordance with an embodiment of the present invention.

FIG. 13C illustrates a method for narrowband data processing using apeak detector in accordance with an embodiment of the present invention.

FIG. 13D illustrates a method for narrowband data processing using aphase tracking loop in accordance with an embodiment of the presentinvention.

FIG. 13E illustrates a method for narrowband data processing using crosscorrelation in accordance with an embodiment of the present invention.

FIG. 14A is a logical system diagram illustrating an alternativeembodiment for hybrid spectral compression and cross correlation signalprocessing within an interceptor as described in FIG. 12 in accordancewith an embodiment of the present invention.

FIG. 14B is a logical system diagram illustrating an alternativeembodiment for hybrid spectral compression and cross-correlation signalprocessing wherein carrier and telemetry signal processing channels areadded, eliminating the need for external configuration data inaccordance with an embodiment of the present invention.

FIG. 15A illustrates a method for acquiring and producing directsequence spread spectrum observables using a combination of spectralcompression and cross-correlation methods in accordance with anembodiment of the present invention.

FIG. 15B illustrates a method for identifying and associating SCPobservables of an intercepted emission with the source emitter using thetechnique of spectral matching in accordance with an embodiment of thepresent invention.

FIG. 15C illustrates a method for identify and associating SCPobservables of an intercepted emission with a source emitter without apriori configuration information in accordance with an embodiment of thepresent invention.

FIG. 16 illustrates a method for extracting carrier observables giventhe SCP code phase observables and basic characteristics of the signalstructure in accordance with an embodiment of the present invention.

FIG. 17 illustrates a detailed example of the hybrid spectralcompression and cross correlation systems using digital signalprocessing approach amenable to software defined radio platforms inaccordance with an embodiment of the present invention.

FIG. 18 is a logical data flowchart of detailing the process flow forapplying two or more sequential delay and multiply operations on a givenbaseband RF signal to produce narrowband data.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

There are situations in which a GNSS implementation for determining thephysical state of some sensor is impractical because the satellitessignals are either too weak, obstructed or interfered with by accidentor intent. Such situations can occur in an enclosed space such as withina metal constructed warehouse, below ground/rubble, or possibly GNSSjamming environments.

By way of overview, the present invention utilizes a beaconconstellation environment, which although low in transmitter power (<1microwatt), provides signal flux that is 40 to 60 dB more powerful thanGNSS signals and thus is able to determine the physical state of asensor in missions where GNSS is either absent or unreliable in thecontext of a configured environment or, in other words, an environmentin which there is the ability to deploy a beacon constellation in amanner that affords the maximum of flexibility for the system operator.The constellation of beacons uses spread spectrum techniques without theneed for time and frequency synchronization while achieving sufficientlystable frequency control to identify a beacon individually by itsfrequency offset. Such beacon constellations could be in terrestrial,marine, air or space environments.

For example, in a terrestrial situation where interference, by accidentor intent, has rendered the GPS (a type of GNSS) unavailable, unmannedaerial vehicles, UAVs, balloon-borne or rocket/parachute beacondeployments may be used. Spectral compression modes are preferably usedwithin the GNSS sensors with high dynamic range digital sampling totolerate residual interference at altitude. In this embodiment, thespectral compression GNSS data are down linked via a communicationschannel or, alternatively, imbedded within the beacon spectrum. In thismanner, the dynamic physical state of these airborne beacons can bedetermined.

Beacons are devices that emit a loosely constrained signal structurethat are configured to simplify the overall design to minimize cost ofan intercepting device, minimize data cross-link requirements andsimplify physical state estimators. The concept of these beacons is notconstrained to operate in any one emission modality. In alternativeembodiments, these beacons operate in several physical domains such aselectromagnetic (RF, optical or nuclear regions of x-rays and gamma) andacoustical (through water, air or solid materials).

The beacon modulation in the preferred embodiment utilizes spreadspectrum full carrier suppression to accomplish code division multipleaccess (CDMA) simultaneous reception of many beacons. The modulationfrom all beacons may or may not be phase coherent or time synchronizedbetween the entire beacon constellation. The constellation signalcoherence and synchronization state is an issue of the choice to be madeby the particular configuration desired and matter related to cost andflexibility of the remote receiver equipment.

The preferred design philosophy is a combination of the satellitenavigation architecture of three segments and the spectral compressionGNSS reception methodologies. The wideband RF signal structure minimizesthe spectral density and the potential for interference with other RFequipment that may be in the area as well as limiting the potential forinterference to the system of this invention. This is preferablyaccomplished by spreading the signals over the maximum band allowed,approximately 20 MHz, by utilizing predefined ranges of ISM bands, forexample, centered at 915 MHz, 2.4 GHz and 5.8 GHz in accordance withcurrent U.S. regulations.

System and Method Overview

The preferred embodiment provides a local area positioning system andmethodology that produces high accuracy positioning (centimeters ifrequired), simplicity of operation and low-cost implementation so as toachieve a ubiquity of utilization. More specifically, the presentinvention blends three methodologies: radio astronomy space geodesy,spread spectrum communications and the methods of non-linear processingof signals from the GPS.

Radio astronomy, such as very long baseline interferometry (VLBI) spacegeodesy, utilizes the concept of an array of incoherent radio sources,typically quasars, to serve as a frame of reference to determine thethree dimensional vector separations between two or more radiotelescopes.

Spread spectrum CDMA communications exploits the methodology of directsequence pseudo random noise (PRN) generation using a linear tappedshift register feedback digital generator. PRN generators use aninternal frequency source to operate the clocking of the shift registeroperation that serves to achieve carrier signal suppression and spreadsthe signal to reduce the spectral density. This provides simultaneousadvantages of channel reuse, relative immunity to in-band interferenceand low probability of detection and interception.

The methods of non-linear GPS signal utilization provide the basis for aderived methodology known as spectral compression that minimizes expensein terms of custom chip/firmware development and DC power consumption. Atypical GPS receiver functions by having a priori knowledge of the PRNcode sequence that each satellite used to spread the carrier signal ontowhich telemetry is modulated. This in turn allows the GPS receiver toextract the navigation message including the time and frequencysynchronization state of each satellite in order for the GPS receiverinternal processor to derive its position and velocity in an autonomousmanner. By comparison, spectral compression GPS methods derive phaseranging data types from multiple synchronized satellites without anyknowledge of the PRN code sequence used to spread the carrier signals.

The design of the beacon constellation avoids the need for time andfrequency synchronization while still functioning as the frame ofreference for physical state determination. In the simplest form, thebeacons form an incoherent array of low power RF signals of very lowspectral density so as to avoid interfering with other systems in thesame spectral region, most likely the ISM bands. The incoherent beaconarray is usable in the differential relative positioning approach of theVLBI. The beacons and location sensors depend upon crystal referencesources no better than those used in inexpensive digital wristwatches,with a frequency accuracy and stability of approximately 10 parts permillion (PPM). In the spectral compression methodology there is notelemetry extraction. As a result, beacons are distinguished from oneanother by their designated frequency offsets relative to PRN sequencechipping rate nominal frequency.

The location sensors do not depend upon cross-correlation signalprocessing of known PRN code sequences to derive pseudo ranging.Spectral compression methods allow the acquisition of ambiguous phaseranging observables derived from a delay and multiply non-linearprocessing that recovers the chipping frequencies of each beacon.

Each of the beacons preferably makes use of the same PRN sequence. In apreferred embodiment, the PRN code is of maximal length, meaning that ithas an auto-correlation function that is zero for all shift valuesexcept when shifted by zero or a value equal to the code length given by2n−1, where n is the number of shift register stages.

With calibration processing of all non-repeated pairs of inter-beaconbaseline vectors, the present invention combines the N beacons into theequivalent of a geodetic network adjustment of dimensions n/2×(n−1)combinations. For example, with six beacons configured to receive ortransmit in accordance with the calibration methods described in thepresent invention, there will be fifteen unique baseline vectors in thenetwork. Network based calculations results in advantages related todata processing, especially when RF multipath contamination is present;for example, multipath contamination will be particular to each of thebaseline vectors and not systematic throughout the network. Thus, thenetwork adjustment produced as a result of the present invention iseffective in deriving the best estimate of the true beacon physicalstate and provides a figure of merit as to the accuracy of theindividual measurements when applied to measurements made by locationsensors. These network estimates can be applied to continuously monitorthe configuration data integrity, making the system self calibrating andable to monitor for unexpected changes in physical states of beaconsrelative to the common internal frame of reference. In the presentinvention, the location sensor physical state may be estimated as partof the network or after application of network adjustments ascorrections pursuant to the a priori beacon Almanac information.

By way of example, various alternative embodiments of the presentinvention are contemplated and illustrate in part the scope andapplicability of the technology.

A centralized processing unit that receives the spectral compressionobservables for one or more location sensors and reference pointsenabling physical state estimation of selected location sensors andreference points.

Placement of the beacons can be somewhat arbitrary, as they themselvescan act as a location sensor, positioning themselves within the networkin a post deployment calibration mode. In this embodiment, vertical inaddition to horizontal placement of at least one beacon device is usedto achieve 3-D positioning.

The location determination system may be underlain on existingcommunication bands without interference. This embodiment utilizeswhatever system exists to augment its capabilities without requiring theexistence of a particular communication network.

Simultaneous observation of beacon signals from a reference locationsensor and from a second location sensor in which a differential signalis formed which removes common time offsets. In this embodiment, timingrequirements are reduced without sacrificing overall measurementprecision while simultaneously enabling a low-cost oscillatorimplementation. CDMA signals are separated in their PRN chippingfrequency with sufficient separation for unique identification. There isno need for a frequency standard better 1 PPM accuracy such as atemperature compensated crystal oscillator (TCXO). In an alternativeembodiment, meter level accuracy location determination is achievablewith low-cost oscillators that are accurate to approximately 50 PPMalthough a proportionally larger separation between the beacon chippingfrequencies will be needed.

Each beacon transmits a spread spectrum CDMA (code division multipleaccess) modulated signal over multiple channels, which are essentiallyoverlapping but with each beacon having a slightly different chippingfrequency for its PRN (pseudo random noise) sequence generator. Theprocessing approach does not require beacon reference frequencycoordination, phase coherence or time synchronization between multiplebeacon units.

Ranging signals within a specified RF band are modulated with a verylong period (on the order of 100's of days) tapped feedback shiftregister sequence, allowing for 100's of simultaneous beacons to operatefrom a given code generation. Each beacon is offset in time within thelong sequence so that it only provides its portion of the sequence overan interval of 1 day. In one alternative embodiment, an approximatelythree second repeating PRN code sequence is used in all beacons, whichhas a chipping frequency of 10.23 MHz with each beacon started at anarbitrary time. This embodiment exploits the fact that there is a lowprobability of ever having two identical start events that coincide andremain within 50 nsec. The identity of the particular beacon, within theconfigured environment, is indicated by the PRN sequence chippingfrequency. For example, an offset of 125 Hz above the nominal 10.23 MHzchipping frequency might correspond with the beacon placed in thenortheast corner ceiling location of a large warehouse.

A location sensor within the domain of the local positioning systemdetermined by the beacons that will despread the CDMA signals utilizingtechniques of Spectral Compression, which recovers the chippingfrequency of the particular beacon being received. Each beacon will usetwo or three PRN channels with different chipping rates (for example,10.23 MHz, 1.023 MHz and 0.1023 MHz, corresponding to ambiguitywavelengths of approximately 29 m, 293 m and 2.93 km, respectively) soas to allow the resolution of phase ambiguities of the next highestfrequency chipping frequency. Frequency offsets, chipping rates, andchannels are all configurable based on the intended application, deviceenvironment, and accuracy requirements, and are fully configurable. Inthe preferred embodiment, the location sensor utilize FFT processing todetermine the amplitude, frequency, and phase for each of the threechannels from each beacon signal received. An alternative embodiment mayalso extract amplitude, frequency and phase using a series of phasedlock loops, one for each beacon on each channel.

With a sufficiently high signal to noise ratio, a single additional102.3 kHz channel may be sufficient to resolve the 29.3 m ambiguity fromthe 10.23 MHz channel. For example, with a receiver operating in aspectral compression delay and multiply mode, that achieves an amplitudesignal to noise ratio of 100 to one, the phase noise will be 0.01radians or 0.6 degrees or 1.6 milli-cycles or 5 meters. A five meterprecision obtained from the 102.3 kHz chipping rate channel willreliably resolves the 29.3 meter ambiguity. The 102.3 kHz channelambiguity will have its 2.93 km ambiguity, however, for a physical spacewhere the separation between the user remote unit is also less than 1.4km, there is no ambiguity. In an alternative embodiment, a third channelof perhaps 1.023 kHz with a 293 km ambiguity and phase precision of 500meters may be used to resolve the 2.93 km ambiguities from the 102.3 kHzchipping frequency PRN generator.

The technology has application for RTLS applications in which locationsensors are placed on an asset to be tracked, and further inapplications such as bar code scanners in which the scanner unit itselfacts as the location sensor, and correlates position to the bar codeidentification of a given asset.

These and other embodiments of the present invention provide some or allof the following advantages:

The capability to arbitrarily place beacons and for them to be able todetermine their own locations, thus reducing the cost and complexity ofinstallation and use of the system.

The capability to eliminate the requirement for time and frequencysynchronization such as between the tags and readers in other systems.This greatly reduces the complexity and cost involved in this system'sdeployment. This flexibility dramatically opens up the possibilities fordeployment in non-standard configured environments such as emergencieswhere search and rescue missions require a timely response.

Use of a distributed architecture in which computation and processing ofdata occurs when appropriate. In one embodiment of the presentinvention, this occurs at a central site with data transferred fromindividual units. In an alternative embodiment, this occurs within thesensing unit itself. The capability of the present invention todynamically locate the computation algorithms allows for simple andrelatively inexpensive implementation of sensors where appropriate, ormore complex and expensive sensors with full positioning capability ifthat is appropriate for other applications.

The capability to perform a hybrid local area and wide area locationdetermination in the same platform. That is, local positioning performedwhen GNSS signals are not available or, if GNSS signals are available,processing data simultaneously.

The use of a software defined radio architecture that allows thesimultaneous processing of GNSS or other signals of opportunity withoutsignificant changes to hardware or software implementation.

Preferred System Architecture

In the present invention, the functional components comprising thephysical state determination system for configured environments can beimplemented in a variety of ways to optimize performance. FIG. 1 showsthe logical functions of the present invention without consideration fora specific implementation or deployment scenario. The diagram shows thefundamental blocks and data relationships typical in a preferredimplementation of the present invention.

More specifically, with reference to FIG. 1, the preferred embodiment ofthe present invention is described as follows. Beginning with aplurality of ranging signal transmitters (RST) 101, the system transmitsmultiple ranging signal transmissions 108 that are simultaneouslyreceived by one or more spectral compressor and translators (SCT) 103.The RSTs preferably transmit one or more ranging signals into asurrounding medium, typically free-space by RF signal, perhaps in theISM bands, although other media are also possible such as by acousticsignal through water, soils, rock or structural materials. Thesealternative signals preferably have characteristics that can beoptimally configured for a particular environment. Each SCT 103 receivessignals from multiple RSTs 101 and processes the signals to produceobservables 110 containing information useful for estimating the SCT'scurrent physical state (for example, position, velocity and time). Oneor more of these SCTs are designated as a reference SCT 104 whoseobservables 111 are used for purposes of system calibration and control.

Continuing in reference to FIG. 1, the observables 110 from an SCT arepassed to a navigation processor 105 together with reference observables111 and almanac and corrections data 112 through a communications means.In the preferred embodiment of the invention, it is not necessary tophysically co-locate the navigation processor and SCT functions as thedata communications between blocks are relatively minimal and can behandled by one or more forms of communications, for example, Ethernet,WiFi (802.11), Zigbee (802.15.4), or any communications medium capableof data transfer. The navigation processor 105 uses the observables thatmay include observables 118 and 111 with the almanac data/corrections112 to determine the physical state estimate 118, which includes atleast one of position, attitude, clock, and temporal derivatives for theepoch(s) specified. Epochs may be the time specified in the observablesor past or future epochs if the navigation processor uses a suitablemodel for propagating state variables forward or backward in time. Thephysical state estimate 118 may be reported to any interested party asdefined by a particular implementation of the system.

The system controller 102 serves to coordinate and monitor the functionsof the system. It receives observables 111 from one or more referenceSCTs 111 via a communication signal. This information may includeoptional external time reference 116 and optional coordinate systemreference data 117, which is preferably collected and passed along tofunctions 106 and 107 for the purposes of producing system configurationand calibration information of past, current, and future physical stateand configuration. The system configuration data 115 is used by thesystem controller to configure and adjust the plurality of RSTs 101 viacommunications signal 119. Communication 119 between system controller111 and RST 101 is optional in environments where the RST 101 rangingsignal transmissions 108 are intercepted by at least one reference SCTenabling the system to determine the physical state of RST 101 by meansof the reference network processor 107. The reference network processor107 uses the collected observables and a priori information about thesystem configuration to compute the physical state of all RSTs 101 andreference SCTs 104 in the system relative to each other. These physicalstates preferably consist of estimates of position, velocity (typicallyzero), clock and clock terms (bias, rate, etc.) as well as RSTtransmission characteristics, which are combined to form the almanac andcorrections data 114. The almanac and corrections data 114 for one ormore epochs are stored in a database 106, which is preferably configuredto provide these data upon demand. In alternative embodiments, theformat of the almanac and corrections data 114 enables efficientcomputation of future states through one or more propagation models. Thealmanac and corrections data is used both by the system controller 102and navigation processors 105 as previously described. In the preferredembodiment of the present invention, the almanac and corrections data114 contains both the estimated state vectors for each RST and referenceSCT as well as additional coefficients for a propagation model thatenables the almanac and corrections data to be used successfully in thefuture. The ability to propagate almanac and corrections data into thefuture is dependent upon the quality of the RST/reference SCToscillators, desired precision and propagation model complexity.

Integrated Wireless Data Communications Configuration

The preferred embodiment of the present invention facilitates areduction in manufacturing cost and complexity of units implementing theSCT function while maximizing flexibility and performance. A furtheradvantage of the present invention is achieved through integration ofsystem functionality with wireless data communication functions, whichallows sharing of digital signal processing and RF front-end circuits.As described in greater detail below, the SCT function of the presentinvention significantly reduces complexity and thus cost as compared tomost wireless data communication receivers. By implementing SCTfunctions as an extension to the communications functions, physicalstate determination capabilities are added with little additional cost.Further, the integration with wireless data communications occursnaturally by combining sending/receiving data functions into the systemcontroller.

FIG. 2A shows the integration of the present invention with a meshedwireless data communications network such as Zigbee (802.15.4). An SCT103 and wireless data transceiver 204 are combined to form an SCTcommunications unit 201. In its simplest form, the unit 201 represents atag capable of RFID and physical state sensing. A beacon unit 202 ispreferably comprised of an RST 101, SCT 104, and a wireless datatransceiver 204. A plurality of beacon units is deployed over a physicalarea to provide both positioning ranging signals 108 and communicationsnetwork infrastructure 205 and 206. The integration of an SCT 104 withthe beacon unit enables each beacon unit to act as a reference SCTcollecting observables from other beacon units deployed within range.Through this combined ranging transmission and collection of observablesthe system facilitates collection of the information necessary todetermine its own configuration using the reference network processor107. In one embodiment, the system controller 102, navigation processor105, reference network processor 107 and database 106 are combined toform a system control unit 203 that centralizes the complex dataprocessing and management functions. The system control unit 203 ispreferably connected to the wireless data network 205 via one or morebeacon units through a communication signal 207. For wireless datanetworks supporting meshed networking, beacon units 202 become nodes inwireless data networks 205 and 206. Meshed network deploymenteffectively simplifies installation of the location system enabling eachbeacon unit 202 to coordinate with the system control unit 203 via otherbeacons units without requiring installation of other communicationmediums (e.g. Ethernet). In the preferred embodiment of the presentinvention, the system control unit is physically connected to one ormore beacon units via an Ethernet connection, which provides advantagesof robustness and reduced cost. For greater portability and flexibility,the communication signal 207 may be accomplished by connecting awireless data transceiver 204 directly to the system control unit 203.

Once deployed, as integrated with a wireless data communications network(shown in FIG. 2A), the present invention can also be used for a varietyof data networking applications between communication devices 208 andnetworked services 209 external to the system. As discussed in furtherdetail below, the communication requirements for the present inventionminimize the need for communications resources, leaving the bulk of thebandwidth available for other activities. In the preferred embodiment,the system control unit 203 is a gateway for networked services toaccess devices on the wireless data networks 205 and 206. The wirelessdata networks 205 and 206 may be secured by data encryption and othersecurity means such that only authorized user is able to access and usethe beacon unit 202 and system control unit 203 gateway infrastructurefor relaying information between devices and services.

FIG. 2B shows an alternate embodiment of the SCT communications unit 201where the navigation processor 105 is integrated directly with the SCT103 and wireless data transceiver 204 functions. This configurationenables calculation of SCT state vector 118 at the unit in situationswhere almanac and data corrections 112 are available from the system.The almanac and data corrections 112 are delivered to the SCTcommunications unit 201 a priori or on demand as requested by the unit201. In an alternative embodiment, the unit 201 may request observablesfrom one or more reference SCTs to determine a full differentialsolution. Similar to the configuration of FIG. 2A, the semi-autonomousconfiguration described in FIG. 2B may utilize system control unitdetermined physical state estimates as needed. For example, thiscapability may be useful in situations where the navigation processor105 is unavailable due to limited power resources.

FIG. 2C shows an alternate embodiment of the SCT communications unit 201in which a machine to machine interface (MMI) 235 is integrated withcore SCT functions 103 and wireless data transceiver functions 204 toprovide SCT physical state estimate (PSE) 118 and data communications233 for external devices 234. This configuration is typical of alocation-enabled communications peripheral, where the external device234 includes custom driver software enabling it to access physical statedetermination and communications functions of the SCT communicationsunit 201. This configuration represents a low-cost implementation withrespect to SCT communications unit complexity. In this embodiment, theobservables 111 are processed by the system control unit 203, whichreturns the resultant physical state estimate 118. This information isrelayed by the SCT communications unit 201 to the external device 234via the MMI 235.

FIG. 2D shows an alternative embodiment of the SCT communications unit201 where both a navigation processor 105 and an MMI 235 are integratedwith the core SCT functions 103 and wireless data transceiver functions204 to provide a semi-autonomous positioning capability. Similar to theembodiment shown in FIG. 2B, this embodiment is capable of determiningthe SCT physical state estimate (PSE) 118 in situations where thesystems control unit 203 delivers appropriate almanac data andcorrections 112. As with FIG. 2C, the SCT communications unit 201provides PSE 118 and data communications 233 to an external device 234.

FIG. 2E shows an alternative embodiment of the SCT communications unit201 with an external device 234 where the navigation processor 105 ishosted by the external device. In this case, the external device hassufficient processing capability to perform the navigation processingfunction enabling the SCT communications unit 201 to be substantiallysimplified, incorporating SCT functions 103, wireless data transceiverfunctions 204 and MMI functions 235, thus requiring less power. Thesystem control unit 203 provides almanac and corrections data 112,and/or processing of the observables 111 to produce the PSE 118 asrequested by the external device 234 in cases where the device opts todisable its own navigation processor 105 function.

FIG. 2F shows an alternative embodiment of the beacon unit 202 where aGNSS sensor capability may be provided using a separate GNSS sensorfunction 240. For example, a separate GPS C/A code correlating receivermay be integrated with a beacon unit, providing an immediate source oftiming and geodetic positioning information about the unit, tying thelocal time and coordinate system to universal time coordinated (UTC) andthe world geodetic system 1984 (WGS-84). The GPS integrated beacon unithas value as a WGS-84 reference point and in facilitating deployment ofthe present invention over larger outdoor areas, where performance canbe significantly improved by using the present invention concurrentlywith GPS.

Integrating the present invention with a wireless data communicationsnetwork, for example as illustrated in the previous series of diagrams,provides flexibility to configure more optimal implementations forspecific applications. One example is the case where a beacon unit isconfigured without integration of an SCT or a wireless data transceiver.This simplified beacon transmits a ranging signal in accordance withconfiguration data loaded prior to its use. These beacons can bedeployed at known points for the purposes of augmenting the positioningperformance when additional communications infrastructure is notrequired. This simplified beacon embodiment is substantially lessexpensive to produce than a more fully integrated alternative.

Integrated GNSS Configuration

The present invention can be easily adapted to simultaneously supportranging signals from GNSS as well as the local signals transmitted by aplurality of RSTs. FIG. 3 shows a logical function block diagram whereGNSS sensing is incorporated with the present invention. The functionsof the present invention previously referenced as 102, 103, 105, 106,and 107 are extended to support reception, processing, and management ofadditional observables and almanac data needed to process GNSS rangingsignals. In this embodiment, the SCT 103 receives both the GNSS 303 andRST 101 ranging signals simultaneously on two separate channels eachconfigured to support the specific characteristics of the ranging signaltype (either RST or GNSS such as GPS). The SCT generates observables 110and tags the data with channel configuration data such that theinformation can be readily processed by the navigation processor 105.The navigation processor is preferably extended to support simultaneousprocessing of both RST and GNSS observable data. Observables may beprocessed in the local coordinate system or some earth-fixed coordinatesystem such as WGS-84. As with the non-GNSS supported implementation,the navigation processor produces one or more physical state estimates118 for each of the SCT observable sets.

To support processing of GNSS observables, the system managementfunctions including components 102, 106, and 107 in FIG. 3 are extendedto manage GNSS constellation information such as satellite orbits, clockinformation, status, etc. The GNSS constellation and observables 301information are collected by the GNSS reference receiver 302 or providedby some external source (not shown) and submitted through acommunications signal 304 to the system controller, which formats thesedata for internal use and stores it in the database 106. The almanac anddata correction 112 provided to the navigation processor is extended toinclude information about the GNSS constellation and current GNSSobservable corrections in addition to the RST almanac and correctionsinformation already provided. In cases where the GNSS receiver is partof the beacon unit discussed previously (FIG. 2E), both the GNSSobservables and the beacon constellation information may be used by thereference network processor 107 to further refine the placement of thebeacons and ultimately improve system precision and accuracy.

Ranging Signal Transmission

While there are a variety of ranging signal structures that can be usedto implement the present invention, the preferred embodiment of thepresent invention focuses on selecting signals that meet the followingcriteria: (1) include necessary precision requirements; (2) can beeasily generated; (3) can be configured to transmit in a variety of RFor acoustic regimes; (4) are resistant to multipath and noise; and (5)possess low interference characteristics compared to other RST rangingsignals in the energy emission region. In the preferred embodiment,direct sequence code division multiple access (CDMA) spread spectrum isthe preferred method for generating ranging signals, where the pseudorandom noise (PRN) sequence is a maximal length code selected for itslow cross-correlation and autocorrelation properties.

In the preferred embodiment, beacon transmissions incorporate codeorthogonality so that significant inter-modulation products will notoccur in the delay and multiply function of the spectral compressor. Thecode properties are available from the GPS gold codes but are typicallylimited by the 32 or 34 code sets. However, alternative code modulationapproaches are possible such as how the GPS design of the P(Y) channelis structured using a very long code sequence of 267 days, which has a10.23 MHz chipping rate. In the P(Y) channel example, seven-day segmentsof this very long code are assigned to each satellite of theconstellation with the entire satellite constellation resetting thephase of the code sequence to its starting condition at midnight eachSaturday. This P(Y) code has the properties of code orthogonality suchthat the auto-correlation of the code is zero everywhere except when thecode shift is zero or by multiples of 267 days. In the presentinvention, any long code with minimal auto-correlation, including theP(Y) code generation, can be configured, after which segments areassigned to each of the beacons.

Many beacons can be operated at random start times and the crosscorrelation between these beacons is essentially zero. For example, a 25stage tapped shift register feedback pseudo random noise (PRN) sequencegenerator will have a code length of approximately 34 million chips codelength. Assuming a chipping rate of 10.23 MHz, it will take 3.3 secondsto repeat this code.

FIG. 4A shows a logical function block diagram of the ranging signaltransmitter (RST) 101, which embodies the signal generation functionsdescribed above. The RST uses a multi-channel ranging signal generator406 to generate the specific ranging signal in accordance with thedesired characteristics. This signal is then used to modulate 404 anintermediate frequency generated by a signal synthesizer 405. Dependingupon the configuration, the resultant signal is filtered by 408 (eitherallowing the upper band, lower band, or both to pass) and passed throughto a digital to analog converter 410. The resultant analog signal isup-converted 411 to the R.F. band using the frequency generated bysignal synthesizer 409. The up-converted R.F. signal is passed through ahigh pass filter 412, amplified 413, and transmitted. The RST controllermanages the particular configuration 403 of the RST module. Each of themodule functions is preferably programmable, which provides theadvantage of enhanced flexibility. An RST may be programmed to transmita variety of different ranging signal structures at various RFfrequencies. This logical structure for the ranging signal transmitterhas many possible variants depending upon the particular implementationdesign and desired optimizations. The preferred embodiment for the RSTis to balance cost, precision and flexibility.

FIG. 4B shows the logical function blocks for a multi-channel rangingsignal generator 406. In this embodiment of the present invention, thegenerator has two programmable channels 432 and 436 that drive a digitalquadrature phase shift keying (QPSK) modulator modulating the I.F.signal generated by the digital signal synthesizer 433. The output ofthe modulator is the digital spread spectrum ranging signal 438 centeredat the I.F. frequency. Each channel (432 and 436) preferably contains adigital chipping clock 434 that is programmable in frequency and phasethat drives the PRN sequence generator 435. The PRN sequence generatorcan be programmed for variety of different maximal length code sequencesand offsets within the sequence. The first channel 432 is preferablychosen as the coarse channel and the second channel 436 as the precisionchannel. Channels 433, 434 and 436 are tied to a common externaloscillator reference to ensure phase coherence. The controller 430manages the generator configuration and provides a simplified interface431 for configuring the function.

Ranging Signal Processing

FIGS. 5A and 5B illustrate internal functions of the SCT 103 describedpreviously, which is the preferred embodiment for processing rangingsignals into the observables needed to determine physical state. In thisembodiment of the present invention, the SCT processes direct sequencespread spectrum ranging signals such as the RST 101 ranging signals andranging signals transmitted by GNSS satellites (e.g. GPS C/A and P(Y)L1/L2 transmissions) simultaneously. The method shown in the illustratedexample utilizes spectral compression techniques that allow suitablyconstructed ranging signals to be compressed into observables (e.g.,amplitude, frequency, phase and time reference) without requiringcomplex cross-correlation signal processing methods that are common intypical spread-spectrum communication systems. With a single channel,the spectral compression method allows for simultaneous compression ofall ranging signals with common characteristics into a set ofobservables. The SCT can preferably implement multiple channels enablingcompression of multiple types of ranging signals in the same ordifferent bands simultaneously. Through this mechanism, the function iscapable of receiving and processing both RST and GNSS ranging signalssimultaneously without loss of continuity as an SCT transitions from oneenvironment to another.

Though spectral compression is the preferred embodiment for processingintercepted emissions, alternative embodiment of the present inventioncan use similar methods of cross correlation, such as GPS, to producecode-phase observables for beacons and GNSS satellites. Using the typesof sensors necessary to produce such code-phase observables would bemore complicated and expensive to implement; however, in certainapplications, such alternative methodology may be desirable if, forexample, needs require that the sensor be able to decode informationembedded within the ranging signal transmission.

In FIG. 5A, suitably constructed ranging signals or any suitable energyemission are intercepted by the spectral compressor translator (SCT) atan RF antenna 504 that is connected to the SCT front-end 501 that iscomposed of a low noise amplifier (LNA) 503 and an RF-down conversionstage 502. As needed, multiple front-end(s) 501 may be implemented tosupport multiple bands. For example, an SCT may be configured to supportone RST/ISM band centered at 915 MHz and the GPS L1 band centered at1575.42 MHz or the L2 band centered at 1227.6 MHz. The output offront-end(s) 501 is an analog signal that is input to an analog todigital (ADC) stage 505 that provides a digital intermediate frequency(I.F.) output. As discussed in greater detail below, it is preferredthat the ADC have sufficient dynamic range to accommodate multiplebeacons of widely different signal levels. The digitized I.F. signal 506is passed to one or more SCT channel processors 507, which produceobservables 513 for physical state (e.g., navigation) processing. Boththe RF front-end(s) 501 and SCT channel processor(s) 507 are controlledand synchronized by the SCT controller 508. This SCT controllercommunicates via control messages 509 to the RF controller 501, and tothe SCT channel processor(s) 507 via channel configuration messages 510.Multiple SCT channel processors may be used to fully capture allavailable positioning observables provided by the ranging signals. Forexample, an SCT configured to operate in both the ISM band and GPS L1may operate five SCT channel processors assigned to one of the followingranging channels: ISM RST coarse channel, ISM RST precision channel, GPSL1 C/A channel, GPS L1 P(Y) channel and GPS L2 P(Y) channel. Each ofthese channels produces observables if the assigned ranging signal ispresent.

FIG. 5B describes the preferred functionality of the SCT channelprocessor 507. The SCT channel processor is controlled by the channeldata acquisition and control function 524, which receives clockinformation 530. The digital IF signal 506 is first processed thru ananti-alias filter 521 to remove spurious or out of band signals. Thefiltered signal output from 521 is sent thru a delay and multiplyprocess 522. The delay and multiply 522 splits the filtered digital IFsignal 506 into two components, one which is in-phase and the otherdelayed by an interval equivalent to one-half of the beacon's spreadspectrum modulation chipping rate (for example, 49 nsec for theprecision 10.23 MHz channel and 5 microseconds for the 0.1 MHz coarsechannel). The delayed signal is mixed (multiplied) with the in-phaseversion 521 signal, which recovers the chipping frequencies of all thebeacons 101. These recovered signals are passed thru a filter/basebanddown-converter 523 where they are temporarily held in a buffer 525. Thebuffered data are processed with a fast Fourier transform 526, and peakscorresponding to the identified beacon signals are identified via a peakdetector 527. The observables from each beacon signal 529 consists of anamplitude, frequency and phase as well as time of observation.

Spectral compression of GPS signals operate because each satellitebroadcasts a unique PRN code so that cross correlation product of eachPRN sequence is essentially zero. Because the Earth is rotating and thesatellites are in twelve hour period orbits, there is a Doppler shiftalong the line of sight of the receiver. From a crude knowledge of timeand the GPS orbits it is possible to predict what Doppler shift isassociated with each individual satellite. Codeless operation, forexample as taught in U.S. Pat. No. 4,797,677, allows for the recovery ofthe chipping frequency of each of the satellites by means of a delay andmultiply operation on the wideband signal from all the satellites. Usinga fast Fourier transform (FFT) processing, each resulting spectral lineis then associated with a specific satellite.

The present invention provides a signal detection method that isavailable compared to a pre-detection wideband signal capture buffer andtransfer for cross correlation detection that is the VLBI approach or apre-detection cross correlation processing of typical spread spectrumsystems. The digital properties of PRN sequences are those having noauto-correlation matches except when the codes are nearly matched(within one half a chip time). For example, if the chipping rate is10.23 MHz, the codes are necessarily aligned within 49 nanoseconds tocreate an interference situation. The same PRN sequences may betransmitted by all the beacons provided that they do not share the samePRN sequence starting epoch and chipping frequency. Neither of theseconditions will likely be achieved with arbitrary starting conditionsand low cost free running reference oscillators.

Accordingly, in a delay and multiply detection approach as taught by thepresent invention each of the spread spectrum beacons are preferablyde-spread into a spectral line at the beacon chipping frequency. Toavoid collapse of the chipping frequency spectral lines into the samefrequency (e.g., 10.23 MHz), each beacon contains its own frequencyoffset value either above or below the nominal 10.23 MHz value. Theoffset magnitude is governed by the precision of the frequency referenceavailable in the beacons. For example, using a reference oscillator withan accuracy of 2 PPM, the frequency is expected to be within +/−20 Hz at10.23 MHz. Given that adjacent beacon channels can be in error by asimilar amount with perhaps an opposite sign so an additional guard bandis required for each beacon. For example, a channel spacing of 50 Hzcould be considered adequate separation given that adjacent beaconchannels could move in opposite algebraic senses and then the beaconswould be separated by only 10 Hz. The frequency offset pattern is set bythe value (50 Hz×N) where N is odd.

In an alternative embodiment of the present invention for high accuracyand robustness, a traditional cross-correlation signal processing schemecan be used in conjunction with the spectral compression methodsdescribed herein. In this embodiment, spectral compression provides themeans to derive physical state information needed to enable rapidcorrelation lock of the correlation channels without searching. Giventhe use of very long code sequences and re-use of the same sequencesoffset in time, the spectral compression method described in thisinvention minimizes the need to implement complicated searchingtechniques. By introducing a cross-correlation capability, particularlyon the precision channel, the present invention takes advantage ofimproved signal to noise ratio and access to carrier phase and frequencydata, which in certain applications (e.g. precision aircraft landingsystems) may be desirable capabilities. However, with the introductionof correlation tracking capabilities, the costs of the receiver sensorare increased significantly and may limit its use when compared to animplementation using only spectral compression.

Navigation Data Processing

The avoidance of high precision time and frequency systems to achievephase coherence of the receiving elements is achieved with the presentinvention preferably by causing all SCTs to observe all beacons duringthe same relative interval. In this embodiment, the FFT time seriesyields one spectral line for each beacon signal received.

By differencing observables from a known reference SCT, the specificphase and frequency offsets of all the beacons are common-mode cancelledin this single differenced data processing in favor of a single offsetof phase and phase rate (frequency offset) of the SCT relative to thereference SCT. In one example, with four or more beacons welldistributed geometrically around the reference and remote SCTs, it ispossible to determine the physical state relative to the reference SCTphysical state.

In an alternate embodiment, equivalent results to those obtained in theabove-described approaches may be achieved by forming almanac andcorrection information at the central reference site by means of thereference network processor or by physical state estimation of eachbeacon relative to at least one reference SCT, and then applying thepreviously computed almanac and correction information during physicalstate estimation given observables from an SCT. This approach ispreferred when the time of applicability for the almanac and correctionsdata is greater than the difference between the time of almanac and theepoch for which the observables of a second SCT are collected. The timeof applicability is a function of the stability of the RST and referenceSCT oscillators, system configuration and the required systemperformance. With the distributed architecture approach, the physicalstate estimation by a navigation processor may take place within theSCT, an RST beacon or at any other convenient location, such as in thecontrol processor.

FIG. 6 illustrates an embodiment of the navigation processor, whichprocesses the observables produced by the SCT and produces physicalstate estimates. The method of this embodiment includes a controlfeedback loop in which solutions from one epoch feed the next. A prioristate information 611 is used to initialize the SCT state vector 601,providing the best estimate of the physical state parameters for theSCT. The SCT state vector 601 is preferably also initialized by the SCTdynamic model 602, which contains information about time varying stateparameters such as time and frequency bias rate, and by the outputestimated state 606 from the previous epoch as calculated by thestabilized Kalman filter 605. The updated state vector 601 is reportedas the physical state estimate 118, which is in turn used to initializethe SCT dynamic model 602 and the RST observation model 604. The RSTobservation model 604 creates the state transition terms required forthe Kalman filter, and also creates the residuals 610 or differencebetween observed and calculated values that are filtered in the Kalmanfilter 605. The RST observation model 604 controls whether data isprocessed in a differential sense with SCT observables 110 beingdifferenced with a reference SCT observable 111, or if SCT observables110 are corrected by combining them with the correction factors 112determined by a reference network. If GNSS data are available becausethe SCT has an unobstructed sky view, the processing proceeds in ahybrid approach in the Kalman filter 605 with residuals 610 beingcalculated in the equivalent GNSS observation model 603. In theseexamples, the SCT observables 110 contain both RST data and GNSSsatellite data, and the SCT observables are used in the GNSS observationmodel 603.

Reference Network

FIG. 7 illustrates an embodiment of the reference network that producesupdate reference point (e.g. beacons or GNSS satellites) almanacs andcorrection data for use by the system in subsequent physical stateestimation for other SCTs. The inputs to the reference network processare a priori system configuration information 705, which is the bestnotion of the state of the system. Actual SCT observables 113, andalmanac data 112, are used to propagate the physical state elements.These are all preferably used to initialize a zone processing filter700, which determines the physical state including position of beaconsand generate almanac and data correction 114 data for the entire networkof beacons within the given zone. As required to optimize efficientcalibration and management of the system, zones may be defined so that agroup several RSTs and reference SCTs are located within proximity ofeach other. Zone based configuration and management enhancesconfiguration flexibility and reduces processing overhead in referencenetwork processing. Within the zone processing filter 700, a singlenavigation processor 105 or multiple navigation processors producephysical state estimate updates for all SCTs. Multiple processors may becombined in a federated filtering sense, in which multiple navigationprocessors 105 concurrently process data sets that have intersectingdata sets. These multiple estimates are combined by a filter combiner702, which creates the composite estimate. The filter combiner 702itself may be a Kalman or other state estimation filter, or may be basedon a statistical combining process. The reference network processor mayalso be responsible for calibrating the network, essentially bydetermining the physical state for all reference points, and reportingthese in the updated state 706. Calibration correction terms arepreferably formatted and stored in a database by the almanac correctionsformatter 703 and are available for use elsewhere in the system.

In one embodiment of the present invention, calibration of zones can beaccomplished by selectively changing the operating mode of the RSTbeacon. Primarily the RST beacon transmits the ranging signal; however,from time to time, it may terminate its transmission so that it canreceive signals using the integrated reference SCT. When operating as areceiver, the RST beacon listens for other transmitting beacons in thezone. Within each zone, multiple beacons may periodically listen toother beacons within the constellation so as to generate additionalobservables that add strength to the estimates produced by the referencenetwork filter. The reference network filter processes these data inorder to update the current almanac state configuration for each beacon.Several methods for managing the beacon operating mode (either transmitor receive) are possible and should balance calibration accuracy withoverall system performance. In the preferred embodiment, enough beaconswould be deployed such that it is possible to simultaneously calibrateand operate the system without adversely affecting performance, orrequired accuracy. A sustained period of initial calibration may berequired when deploying the system for the first time and adding newzones. In these cases, a calibration pattern may be used where multipleRST beacons are cycled from transmit to receive modes such that multipleindependent measurements can be made such that systematic errors arereduced. Once calibrated, the system is monitored and continuallycalibrated using an on-the-fly technique to update oscillator statecoefficients and confirm placement of the beacons. Monitoring alsoprovides useful data to determine the overall health and accuracy of thesystem.

Physical State Processing Methods

FIG. 8 illustrates two methods of determining the physical state for anSCT given an a priori set of almanac and corrections information andobservables from a reference SCT. In FIG. 8A, observables from areference SCT 805 are used to calculate real-time correction 807 thatwhen applied to correct the estimated physical state to be the actualstate as defined by the almanac for the reference SCT 805. Thecorrection vector is used to calculate a physical state correction foreach RST 801, 802, 803, which is then used to correct the physical stateestimation process for SCT-B 804. An alternative but equivalent formusing differential estimation is shown in FIG. 8B. The observablesproduced by reference SCT 820 are differenced with the observablesproduced by SCT-B 821, which is used to calculate the relative physicalstate 822. Adding the relative physical state to the reference physicalstate for SCT 820 produces the physical state for SCTB 821.

For systems where unmodeled error is negligible, these two methods areessentially equivalent; however, the differential method in FIG. 8B willtend to be more precise when unmodeled error is significant due to thecommon mode rejection of error contributions for each RST. Theautonomous method of FIG. 8A may be less accurate but has the advantageof better scalability since observables for the reference SCT's need notbe processed by each physical state estimation. Rather, they can becalculated once and formatted into corrections that are easily appliedto subsequent processing as long as they are applied within the time ofapplicability.

Deployment Configurations

FIG. 9 shows an illustrative example of three-dimensional positioning inwhich SCT units are located by intercepting emissions from RSTs placedin a non-coplanar configuration. In this embodiment, a reference SCT 904intercepts emissions from RSTs 901, 902 and 905, which are in the samehorizontal plane. Additionally, emissions are intercepted by SCT 904from RST 906, which is located in a plane below the reference SCT 904.Additionally, a second SCT 903 intercepts emissions from the four RSTs901, 902, 905 and 906. The fact that the beacons are not necessarily inthe same plane as the SCT sensors allows for vertical and horizontalpositioning of the SCT units 903 and 904, resulting in a threedimensional position given the preferred geometry.

FIG. 10 illustrates one possible deployment scenario of the presentinvention using both locally deployed RSTs together with GNSS satellitesto provide physical state estimation in both GNSS obstructed andunobstructed cases comprising three operating environments: anobstructed GNSS environment, a semi-obstructed GNSS environment and anunobstructed GNSS environment with fringe coverage. FIG. 10 illustratesthe seamless transition from an outdoor wide-area solution using GNSS toa total local-area system where GNSS satellite signals are totallyobstructed. Though simplified to a 2-D illustration for purposes of thepresent disclosure, this illustration of the implementation of anembodiment of the present invention is equally applicable to a 3-Ddeployment. The physical state contains two position state parameters:horizontal displacement and vertical displacement.

SCT-A 1007 operates in the obstructed environment deriving physicalstate estimates using intercepted emissions from RSTs 1005, 1006 and1008 in the manner previously described herein. GNSS satellite signal1002 are either absorbed or reflected by the structure 1013 such thatthe signal level at SCT 1007 is too weak to provide useful observables.A GNSS reference receiver 1003 is deployed on structure 1013 for thepurposes of collecting constellation and observable corrections that arestored in the database (not shown) for subsequent use by navigationprocessors (not shown).

The next situation in FIG. 10 is the semi-obstructed GNSS environmentwhere SCT 1009 receives signals from GNSS and RSTs. In this example, notenough satellites are visible (only two) to derive physical stateestimates; satellite 1001 ranging signals are blocked from view by thestructure 1013. Using the present invention, SCT 1009 interceptsemissions from RSTs 1006, 1008 and 1010 for positioning and with theaddition of the two visible GNSS satellites. This significantly improvesthe accuracy and precision of the physical state estimation. Thesatellite constellation information collected by the GNSS referencereceiver 1003 provides satellite orbit information used to estimate thephysical state using GNSS observables. Accordingly, this embodiment ofthe present invention provides advantages associated with augmentationof GNSS coverage in semi-obstructed environments.

The unobstructed GNSS environment in FIG. 10 is represented by SCT 1011.In this example, GNSS provides adequate coverage (represented here bythree satellites, although additional satellites may be present) toestimate the physical state. Only a single RST 1010 is visible, which isnot enough to produce a useable physical state estimate by itself. TheSCT 1011 collects observable data from the GNSS and RST and utilizes awireless network (not shown) to process the observables into a physicalstate estimate.

Alternative Applications of the Present Invention

In this section, specific applications of the system are presented toillustrate some of the many anticipated uses of the technology. Theseapplications are all possible with the preferred embodiment of thepresent invention; they are illustrative only of alternatives readilytaught by the present invention, and are not meant to define anexclusive set of possible applications.

Integrated Bar Code Scanning Application

An alternative embodiment of the present invention provides forintegration of an SCT communications unit with a barcode scanner. When abarcode associated with an object is scanned, the time and position ismaintained as a record of the last known place and time the object wasobserved. For inventory and warehouse logistics, this application of thepresent invention enables 3-D indoor tracking of items without theexpense of actually tagging the object with its own SCT communicationsunit. Position tagged barcode scans offer an alternative approach toimplementing a full RFID tracking and positioning system where the sizeand/or cost of the tracked asset does not justify the additionalexpense.

Integrated Passive RFID Tag Reader Application

An alternative embodiment of the present invention provides forintegration of an SCT with a passive RFID tag reader. When an RFID tagreader detects a passive RFID tag, the location of the reader at thetime of this detection is associated with the scanned RFID data streamto provide approximate location of the RFID tag. Additionally, a furtherrefined estimate of the RFID tag position can be determined by combininginformation about relative power of the measured tag data with thelocation and attitude of the tag reader.

Indoor/Outdoor Logistics Applications

An alternative embodiment of the present invention provides foradvantages in logistics in intermodal transport, engineering andconstruction. Such applications benefit from real-time tracking andmanagement of assets moving in and out of obstructed environments. Forexample, a Zigbee or GNSS solution integrated as taught in the presentinvention enables broad use of the technology in locating andcommunicating with assets throughout a localized area in threedimensions.

The present invention is also uniquely suited for this application givenits inherent capability for self-configuration and calibration. An SCTcommunications unit no larger than a cell-phone may be used to quicklysurvey multiple points faster than is possible with theodolitetechnology or GNSS alone. Further, working in a similar fashion to alaser level, an SCT communications unit can determine horizontal andvertical alignment of any structural component to the sub-centimeterlevel relative to any desired reference point.

For site logistics, a similar cell-phone sized device (potentiallysupporting voice as well) may provide real-time tracking of people andassets throughout the entire construction site, including to placeswhere a GNSS based solution is unreliable or totally unavailable. Withintegrated telemetry, the system becomes a powerful tool forcoordination and monitoring of site activities. With support for meshnetworking, sites of virtually any shape and size can be easily coveredand managed centrally without the on-going expense of a wide-areawireless solution (for example a GSM/GPS solution).

Healthcare Applications

Alternative embodiments of the present invention may be readily appliedin health care facilities. For example, an SCT communications unitintegrated with either Zigbee or WiFi may provide real-time monitoringof patients and assets. Supervisory and patient services staff need thecapability to locate doctors, nurses, patients and mobile equipmentwithin the hospital facilities. Patients with severe mental illness posea serious challenge if they move outside a geo-fence, and alarms couldbe activated in such situations to restrict the patient's further traveland provide the location of the patient for retrieval by staff. Patientson gurneys can also be easily located-critically important if they spendsignificant time outside of assigned areas, such as during emergencymanagement or in situations when patients exceed hospital bed capacity.Further, with support from GNSS signals, the SCT communications unit cannotify managers when patients leave the healthcare facility boundarieswithout authorization or discharge. This is particularly useful forAlzheimer patient tracking.

Alternatively, another embodiment for healthcare applications would beto equip selected staff members with a portable RFID reader equippedwith an SCT such that the approximate location of passive tags can bedetermined through ad-hoc sampling. In this embodiment, the staffmembers would proceed through normal activity, where the SCT equippedreader would regularly poll for passive RFID tags, any receivedresponses would be tagged with the current time and location ascalculated by the present invention.

Location Commerce Applications

With the combined capabilities for simultaneously processing both GNSSand local area RST signals, the present invention enables high-precisionlocation commerce applications both in obstructed areas and where GNSStypically provides services (e.g. outdoors). An alternative embodimentof the present invention is to equip consumer communication devices suchas cell phones and other mobile devices with SCT functions such thatlocation can be determined both in large geographic regions as well asin localized areas such as a shopping mall. The SCT equippedcommunication device can be used to identify the location of anindividual enabling the delivery of location specific content relevantto the individual's precise location. With minimal cost, the presentinvention performs both wide area positioning and local area positioningsimultaneously, yielding accuracy and positioning information where GNSSalone is unable to function. Unlike current location commerceapplications using GNSS/network assisted location services, thisalternative embodiment of the present invention allows the individual tobe pinpointed with meter level accuracy indoors and outdoors. Further,the present invention can smoothly transition from local areapositioning to wide area GNSS without loss of coverage. For example,given a store that has deployed an array of RST beacon units for thepurposes of position, information regarding the selection of goods andservices in the immediate vicinity can be delivered to an individualwith an SCT equipped cell-phone; this information may includeadvertisements, product information, coupons, purchase statistics, andratings. Further, in this embodiment, the communications network alreadysupported in the device can be used to transport the location relevantcontent.

Emergency Services Applications

In a situation such as post-Katrina New Orleans where there was nosurviving regional communications networks, the present invention withits integrated communications infrastructure may provide a telemetrynetwork and accurate tracking of first responders, vehicles, supplies,and other key mobile assets. In this embodiment, the SCT communicationsunit is integrated with Zigbee and P25 VHF to form a robust local areaand wide area location and communications management solution. Thisembodiment enables real-time monitoring of rescue workers as they enterbuildings during search and recovery and to provide for regionalmonitoring when out of doors (via GNSS). Alarms could be triggered inthe event of the absence of a first responder's lack of movement, whichmay be indicative of an emergency situation.

Aerial Search and Rescue (SAR) Applications

An alternative embodiment of the present invention may be utilized forsearch and rescue operations. In one example, two SCT communicationunits may be deployed into an airborne environment (either free flyersor one flyer and one towed package). Each SCT communication unit isconfigured to process GNSS signals simultaneously with RST rangingsignals. A beacon unit is deployed with a victim that to be located. Thebeacon unit transmits an RST ranging signal that may be receivedoverhead. In certain situations, the victim may be deep within aforested environment, buried in the snow, or in some obstructedenvironment that prevents normal use of GNSS sensors.

The ground segment (GS) consists of a pair of UAV controllers of theseairborne platforms and a Zigbee two-way communications subsystem thatcontrols airborne operations and retrieves the SCT observables from theUAVs. The ground segment also has a conventional GNSS receiver thatallows the acquisition of GNSS orbits and time. A ground processorreceives Zigbee downlinks, determines the dynamic inter SCTcommunication units baseline vector separation, beacon delta phase andderives the intersected hyperboloids that gives the beacon's groundlocation which is associated with the victim under debris (i.e., anavalanche or collapsed building).

These UAVs may be very small type model aircraft, which could beconsidered as expendable assets, depending upon circumstances. A minimumof two UAVs flying in the area of interest are enough to be able to findthe beacon with several meter accuracy after a few minutes of flyingabove the general region of interest. When the SAR team arrives in thegeneral region as indicated by the airborne segment, a hand-heldSCT-type receiver as described in the present invention, can be operatedin a total power detection mode, which will provide meter level accuracyguidance for digging and effecting the actual rescue operations.

FIG. 11 illustrates an alternative embodiment in which the presentinvention is utilized for search and rescue operations. In thisembodiment, an RST signal emitter 1104 is placed with an asset or personto be tracked and located in case search or rescue is required. The RSTbeacon produces ranging signals 1101 that are intercepted by SCT units1102 and 1103 located on unmanned aerial vehicles or other flyingplatforms. Utilizing the techniques described previously herein, rangemeasurements 1106 and 1107 are determined between the flight platforms1102 and 1103 and the asset to be located 1104. The UAVs 1102 and 1103also simultaneously receive data from a GNSS satellite constellation1101, which can be used to determine an autonomous location at the timethat they intercept the RST ranging signals 1101. Each range measurementcombined with location of the observing SCT produces a hyperbolic arc ofpossible location of the emitter. For example, if the location of theUAV 1102 is known from GNSS data 1101, and a range 1106 is determinedbetween the UAV 1102 and the emitter 1104, it is possible to say thatthe emitter is located on a hyperbolic arc of position 1108.Simultaneous observation of a second such arch 1109 can be used todetermine the location of the emitter 1104 that lies on one of the twopossible intersections of these arcs 1108 and 1109. In search and rescueoperations, one of these two intersection points can generally bediscarded as out of plane, and the asset located.

Maritime Station Keeping and Close-Quarters Navigation

An alternative embodiment of the present invention involves tug andbarge towing operations at-sea and during approach to locks. The beaconallows phase-stabilized GNSS sensors on tug, at lock entrance and atmultiple points on barge(s).

The tug would provide the beacon reference signal (perhaps in the 2.4GHz ISM band) to phase-lock the barge GNSS sensors. The tug also has a915 MHz ISM band receiver to receive the primary reference signal fromthe lock, if it was available. The lock also has a GNSS receiver drivenby the lock reference source that is being broadcast to the tug andothers vessels as required. GNSS sensor data is also acquired using thesame ashore reference oscillator. The lock reference signal at 915 MHzwould be used to phase-lock the tug GNSS sensor and then the tugreference beacon at 2.4 GHz, which phase-locks the multiple GNSS sensorson the barges. If the tug is out of range of the 915 MHz ashore lockreference signal, the tug internal reference is the source tophase-locked array of GNSS sensors on the barges. All GNSS sensor data,from ashore, the barges and the tug, are collected and processed at thetug. This phase coherent array is processed in real-time with anaccuracy of better than 30 cm and in the Earth-centered Earth-fixedcoordinate system of the WGS 84. Aboard the tug, position and velocitysituational awareness information can be available at the tug's bridgecontrol. The low cost architecture allows the formation of an affordablesystem that is unachievable by other means.

On-Orbit Operations—Mother Satellite with Orbiter Daughter Satellite

An alternative embodiment of the present invention involves relativepositioning in space of a daughter satellite, which is co-orbiting withanother main satellite at altitudes where GNSS signals are unavailable.Small nano-powered beacons are placed on the mother satellite at knownlocations of opportunity. These known beacon locations form the frame ofreference for positioning of sub-satellites. All of these beacons aretime synchronized and phase-coherent relative to the mother satelliteinternal time and frequency reference source. The daughter satellitemoves around in the vicinity of the mother satellite. The observablesare the phase ranges from the various beacon signals arriving at thedaughter satellite. The observables would be linked back to the mothersatellite for processing. Four or more observables are required in orderto estimate the 3-D position of the daughter satellite and tosynchronize the daughter internal time reference source. Depending uponthe distance separation between the mother/daughter, the GDOP parameterwill be a significant issue because as the daughter will tend to viewthese multiple beacons as a point source at a distance of approximatelytwenty times the maximum separation between the beacons on the mothersatellite. For a five meter maximum beacon separation at the mothersatellite, and with a few millimeter range measurement precision at thedaughter satellite, the 3-D position of the daughter satellite relativeto the mother can be estimated with a precision of a approximately 20 cmat a 100 m separation between these satellites.

Low-Cost 3D Land Survey System

An alternative embodiment of the present invention may be utilized forlow cost land surveying systems. A common beacon is used to phase-lockall GNSS sensors, which cross-link their SCT data to a centralprocessor. The central processor has satellite orbits and GNSS time.Pseudo range and carrier phase data types provide millimeter precisionsover kilometer scale operations. Systematic errors due to multipathcontamination will be limiting error sources for this method and can bemitigated by special GNSS antennas. On short baselines typicallyinvolved in local area construction, the atmospheric errors from thetroposphere and the ionosphere will be common-mode self cancelingerrors. Survey system designs are possible that can reducemulti-instrument system cost by 70% to 90% relative to currentlyavailable instruments.

Precision Takeoff/Landing for Shipboard Rotary Wing Aircraft

An alternative embodiment of the present invention may be utilized forpositioning during takeoff and landing of rotary wing aircraft operatingin shipboard environments. Conventional GPS based tracking systemscontain significant limitations for such applications due to theinability of a conventional GPS receiver to decode the 50 bps navigationdata stream, and due to the potential for interference from othershipboard navigation and communication systems. The technology of thepresent invention mitigates these concerns by placing RST beacons on theship super structure, and SCT receivers on the aircraft. The system andmethod do not require decoding of a data stream to determine beaconposition for operation, and frequency of operation can be adjusted tominimize interference with other systems. Additionally, the rapid updaterate of the present invention handles the relevant dynamics of both theship and the aircraft.

Augmented GNSS Aircraft Precision Approach

An alternative embodiment of the present invention may be utilized foraugmenting aircraft precision approach and landing operations. A localRST network is placed around the runways of a landing strip. SCTs aboardthe aircraft recover beacon data and utilize this data to augmentpositioning from GNSS or other means. The data can be processed in acombined solution, and there is no interference between the RST beaconsystem and GNSS systems because the RST frequencies are adjustable. Thisapplication can be applied to land based aircraft landing strips and toshipboard applications such as fighter aircraft deployment from a Navyaircraft carrier. The high update rate available with the RST beacon andSCT receiver handles the extreme dynamics of such an aircraft.

Yet another alternative embodiment of the present invention is toprovide a rapid deployment and recovery capability for aircraft withoutreliance upon GNSS signals. The embodiment would function withoutreliance upon GNSS signals being available to support air operations. Areference SCT at the airstrip provides RST beacon calibration data,which is up-linked to the aircraft. The aircraft receives theground-based beacons and the reference site calibration data andprocesses an estimate of the position and velocity of the aircraftrelative to the ground based system from several beacons surrounds theairstrip. In this configuration, each aircraft has its own navigationprocessor and remains in an emission silent mode.

The system horizontal positioning accuracy will be limited by the RSTbeacon position calibrations at approximately 10 cm. Because these RSTbeacons will tend to be co-planar, the horizontal dilution of precision(HDOP) will be good at near unity; however, the vertical DOP for theaircraft will be in the domain of a factor of 10 to 20. Because thesystem has high precision of a few centimeters, the aircraft verticalprecision estimate to be within a meter over a broad domain of altitudesas the aircraft approaches the airstrip. Placement of one or more RSTbeacons out of plane with the rest of the beacons will improve precisionin the vertical estimates. As a backup, when the aircraft comes to analtitude of approximately 5 meters, an acoustic RST could be activatedwith an acoustic mode SCT that would provide altimetric accuracy of afew cm and with low probability of detection that will allow theaircraft to flare for touchdown.

The aircraft can also carry three beacon receivers to provide anattitude determination capability. These attitude receiver antennaswould be located on the underside of the aircraft probably at eachwingtip and at the aft end of the fuselage. The aircraft processor wouldcompute the phase differential arrival from each beacon and be capableof determining the aircraft attitude with an accuracy of a few degreesdepending upon the specific aircraft geometry relative to the groundbeacons.

Airport Ground Tracking and Monitoring System

An alternative embodiment of the present invention may be utilized forairport ground tracking and monitoring systems. In this application, thepresent invention will function inside of buildings such as hangers, andin obstructed areas where GNSS navigation alone will be unreliable. Whenan aircraft which has been in an enclosed environment for a significantperiod of time exits the hanger, there may be a substantial amount oftime required for the GNSS receivers to begin positioning. Thisapplication provides aiding data of position and time to such receivers,and thus enhances runway incursion detection and collision avoidancealerting. Further, this application enables centralized monitoring andsecure data base development of tracked assets.

Local Area Location Authentication

In yet another alternative embodiment of the present invention, thesignals transmitted by the RST can be used to authenticate the locationof an SCT by processing the observed data captured by the SCT togetherwith Reference SCT observables to determine if the SCT is at the apriori known location of the SCT. The observables collected by the SCTto be authenticated contain useful information unique to the location(the location signature) that can be authenticated by observing thecurrent state of the RST array via the Reference SCT and the observederrors in the location signature. The fact that a plurality of RSTs areunsynchronized and phase incoherent in their PRN chipping relative toeach other requires continuous calibration of the RST array but bringswith it a security attribute in that an adversary could not predict wellenough the various code phases or chipping rates to achieve sub-meterprecisions. The reference SCT, which is presumed to be protected, willsense and report what is actually happening with the RST array. This isa very useful attribute because these unpredictable features make thepresent invention the way to implement location authentication in GNSSobstructed environments. Additionally, with the present invention'scapability to process GNSS signals, it can also provide GNSS derivedlocation signature data as well.

Design Considerations

The analysis of the transmission power levels, battery consumption,identification and differentiation of beacon signals and othercharacteristics has been carried out for variations of the preferredembodiment. These are detailed in the following sections, which areprovided solely to demonstrate present implementation of various andalternative embodiments of the present invention.

RST Beacon/SCT Receiver Design Considerations

The coarse channel receiver self noise assuming a 3 dB noise figure lownoise amplifier will be: KTB noise power=(1.38×10⁻²³ W/Hz-K)(300Kelvin)(2×10⁶ Hz)=8.2×10⁻¹⁵=−140 dBW=−110 dBm.

Consider a 0.1 micro-Watt (1×10⁻⁷ W) beacon power at a distance of 3 km.

Beacon flux at distance D, P_(rec)=P_(xmtr)/(4 pi D²), P_(rec)=(1×10⁻⁷W)/4 pi (3000)²=9×10⁻¹⁶ W=−150 dBW=−120 dBm.

Beacon signal power=−120 dBm. Post-LNA SNR=−120−(−110)=−10 dB

Delay and multiple (D&M) processor squares the signal & noise so thatSNR D&M=−20 dB.

Assuming a beacon with a 1.023 MHz chipping frequency and an SCT FFTprocessor with a 1 second time series has 1.0 Hz bin width and aneffective Process Gain, Gp=2 MHz/1 Hz=63 dB.

Overall system power SNR=63 dB−20 dB=43 dB or 22 dBV amplitudeSNR=140:1.

The FFT phase noise estimate is the reciprocal of the voltage SNR, sothe phase noise=7×10-3 radians=0.4 degrees=1 milli-cycle.

The beacon with a PRN chipping rate of 1.023 MHz, 293 m wavelength. The1 milli-cycle precision will provide a 30 cm Coarse channel phaseranging precision.

Consider now the precision channel receiver self noise assuming a 3 dBnoise figure low noise amplifier will be: KTB noise power=(1.38×10-23W/Hz-K)(300 Kelvin)(20×106 Hz)=82×10-15=−130 dBW=−100 dBm.

Consider a 0.1 micro-Watt (1×10-7 W) beacon power at a distance of 3 km.

Beacon flux at distance D, Prec=Pxmtr/(4 pi D2), Prec=(1×10-7 W)/4 pi(3000)2=9×10-16 W=−150 dBW=−120 dBm.

Beacon signal power=−120 dBm. Post-LNA SNR=−120−(−100)=−20 dB.

Delay and multiple (D&M) processor squares the signal & noise so thatSNR D&M=−40 dB.

Assuming a Beacon with a 10.23 MHz chipping frequency and an SCT FFTprocessor with a 1 second time series has 1.0 Hz bin width and aneffective Process Gain, Gp=20 MHz/1 Hz=73 dB.

Overall system power SNR=73 dB-40 dB=33 dB or 16.5 dBV amplitudeSNR=50:1.

The FFT phase noise estimate is the reciprocal of the voltage SNR, sothe phase noise=2×10-2 radians=1.2 degrees=3.2 milli-cycle.

The beacon with a PRN chipping rate of 10.23 MHz, 29.3 m wavelength. The3.2 milli-cycle precision will provide a 9 cm precision channel phaseranging precision.

Battery Power Requirements

The beacon power requirements will be dominated by the digital circuitryand not the very low power of the 0.1 micro-Watt beacon transmitted. Thebeacon will require approximately 40 mW assuming 1.8 V logic. Consider a3.3 V Lithium—Manganese battery of 1500 mA hour capacity with thevoltage falling to 1.5 V in 50 hours or about two days. The power sourcecould also be batteries with a solar recharge if in an outdoor situationor powered from conventional building power with a battery backup toprovide for continuous operations.

Beacon Identification

The beacon identification will be by its frequency offset from thenominal 1.023 MHz coarse channel chipping rate with multiples of 5 Hzspacing offsets between beacons. Thus, for a hundred beacons, theprocessor would have a total search interval of +/−250 Hz centered at1.023 MHz. Once a particular beacon chipping rate was identified, theprocessor would refer to the registry data base to determine to whatperson or asset the identified tag had been assigned.

Similarly for the Precision channel the beacon identification will be byits frequency offset from the nominal 10.23 MHz Precision channelchipping rate with multiples of 50 Hz spacing offsets between beacons.Thus, for a hundred beacons, the processor would have a total searchinterval of +/−2500 Hz centered at 10.23 MHz. Once a particular beaconchipping rate was identified, the processor would refer to the registrydata base to determine to what location, person, or asset the identifiedbeacon had been assigned.

ISM Band Implementation

In an alternative embodiment, an RF implementation with each beacontransmitting multiple phase coherent channels of direct sequence spreadspectrum signals is described. For example, to achieve positioningwithin a confined environment where the receiver device is a priorilocation is known within 500 meter, there is a channel with a chippingrate of 1.023 kHz (wavelength of 3 km). With a location sensorimplementing a spectral compression delay and multiply operation and aresultant amplitude signal to noise ratio of 20 to one, the phase noisewill be 0.05 radians or 2.8 degrees or 7.9 milli-cycles or 24 meters.

With a second channel with an SNR of 20 and a chipping rate of 1.023MHz, the phase range precision is 2.4 meters. With a third channel withan SNR of 20 and a chipping rate of 10.23 MHz, the phase range precisionis 24 cm. With a fourth channel with an SNR of 20 and a chipping rate of102.3 MHz, the phase range precision is 2 cm.

The estimated SNR of 20 is very modest and effective SNR at 100 could bemore reasonable. In this higher signal case, the 10.23 MHz chipping ratechannel would yield 5 cm precision. By U.S. regulations, the ISM bandsare:

5725-5875 MHz (150 MHz center frequency 5800 MHz)

2400-2500 MHz (100 MHz center frequency 2450 MHz)

902-928 MHz in Region 2 (26 MHz center frequency 915 MHz)

Beacon locations can be expressed in the WSG 84 coordinate system tomaintain a consistent frame of reference with the GNSS. Thus, theresulting physical state estimates could express the positions in theGNSS frame as if they had clear lines of sight to the GNSS satellites.

Application to Positioning in a Large Area

In an alternative embodiment, application is in reference to an areadefined 100 m by 100 m (10,000 square meters, 110,000 square feet). Themaximum horizontal distance that a location sensor could be away from abeacon is approximately 141 meters. Consider a design for a spectralcompression system with an intercepted phase measurement precision of 3cm. With a maximum chipping rate of 10.23 MHz, there is a 29.3 mwavelength. A 3 cm precision requires 0.1% of a cycle (0.36 degrees)phase measurement precision or 6.3 milliradians. Six milliradian phaseprecision requires FFT amplitude SNR of 160 or 44 dB signal power.

Telecommunications Considerations for the Present Invention

In an alternative embodiment, various test cases may be described.

Test case: ISTAC 2002 Codeless GNSS Land Surveyor

The receiver self noise assuming a 1.5 dB noise figure low noiseamplifier will be: KTB noise power=(1.38×10-23 W/Hz-K)(120 Kelvin)(2×106Hz)=3.3×10-15=−145 dBW=−115 dBm.

GPS C/A channel signal power=−130 dBm. Post-LNA SNR=−130−(−115)=−15 dB.

Delay and multiple processor squares the signal & noise so that SNRD&M=−30 dB.

FFT processor with 40 second time series has 0.025 Hz bin width,effective Process Gain, Gp=2 MHz/0.025 Hz=79 dB.

Overall system SNR=79−30=49 dB or 25 dBV amplitude SNR=316:1 in goodagreement with the actual C/A channel performance of the ISTAC 2002 LandSurveyor product.

Near-Far Degradation in a Warehouse Environment

In an alternative embodiment, a near-far degradation in a warehouseenvironment may be described.

At the nearest, the 1 nano-W beacon might be within 10 m of the remotereceiver.

Beacon flux at distance D, Prec=Pxmtr/(4 pi D2), Prec=(1×10-9 W)/4 pi(10)2=8×10-13 W=−121 dBW=−91 dBm.

A beacon at 141 m will present −114 dBm while a beacon 10 m away willpresent −91 dBm. Thus, the near-far problem is the absolute value of −91dBm minus−114 dBm=23 dB. With 12 bits of analog to digital conversionthe receiver will have 72 dB of dynamic range and allows a 49 dB ofmargin to accommodate other relatively higher power in-band signals thatcould shift the noise floor.

Simplicity of Receiver

An advantage of using a spread spectrum approach for beacons is toradiate the least amount of power, reducing DC power requirements forbeacons that may be battery powered for operations over long periods oftime. The spread spectrum utilization affords a high level of immunityto strong in-band signals that would otherwise present substantialinterference with a conventional signaling modality.

Generalized System Architecture and Method

The previous discussions of the various embodiments of this system andrelated methods for physical state estimation in configured environmentsshow the broad applicability to a wide variety of applications. Thesystem and method disclosed and taught above may be summarized in thefollowing description of a generalized architecture, which reduces thesystem to its canonical form essentially comprised of emitters,interceptors implementing spectral compression and a physical stateestimator and covers most if not all possible implementationarchitectures. The form also teaches that through proper design andconstruction, the preferred embodiment of the present invention can beeasily adapted to support a broad spectrum of applications,configurations, and environments.

FIG. 12 illustrates the canonical form of the preferred embodiment ofthe present invention detailing the essential relationships between thesystems basic elements. At least one or more emitters 1201 are known tothe system, which emit energy that propagates through a transmissionmedium 1206. These emissions are intercepted by at least one interceptor1202 and processed by at least one of the methods of spectralcompression by the spectral compressor 1205. The resultant observables1207 from at least one interceptor are communicated by somecommunication means to a physical state estimator 1203. Configurationdata 1208 and the observables 1207 are processed by the physical stateestimator to determine one or more members of the relative physicalstate estimate 1209 between at least one emitter 1201 and interceptor1202. Observables 1207 from multiple emitters may be used forsimultaneous estimates of multiple members of the relative physicalstate which may include position in the X, Y, and/or Z axis, orientationabout some axis, clock bias, and potentially any time derivatives.

Determining an absolute physical state estimate 1209 requiresdesignation of at least one emitter or interceptor as a reference pointthat has some aspect of its physical state known prior to estimation ofthe relative physical state. Determination of the absolute physicalstate 1209 is the addition of relative physical state to the a prioriphysical states defined by the reference points.

One or more references points defined within the configuration data 1208can be treated collectively to form a local reference frame forpositioning and timing information. Preferably all physical stateestimates 1209 are reported within this reference frame. Further,reference points can be associated 1210 and 1211 with a coordinatesystem fiducial reference 1204 within the configuration data 1208.Through these associations, estimates determined in the internalreference frame can be translated to an external reference frame.

For example, in an indoor applications, a plurality of beacons (e.g.,emitters 1201) are first calibrated such that the combination ofconfiguration data and system calibration data enables the beacons to beestablished as reference points for physical state estimation of alocation sensor (e.g., an interceptor 1202). The location of thesereference points are then determined in the external WGS-84 referenceframe. This can be accomplished in any number of ways through survey, orthrough direct measurement with location sensors supporting reception ofGNSS ranging signal emissions. With these determinations of externalfiducial references a transformation matrix can be specified thattranslates from the internal reference frame to the external WGS-84frame. In the preferred embodiment, three non-colinear reference pointsassociated with external fiducial points are used to establish athree-dimensional transformation. Once this is accomplished, theresultant estimate of physical state for a location sensor can bereported in the external reference frame. Reporting of time epoch ininternal and external time frames such as universal time coordinated(UTC) may be accomplished in the same manner using the time at referencepoints with respect to the external time frame.

Some emitters may be known to the system but not controlled by thesystem and considered external. GPS satellites, quasars, communicationssatellites, television stations and autonomous beacons are all examplesof reference points whose existence can be known and monitored but notmanaged by the system.

In the same manner for defining the canonical form of the systemarchitecture, the related canonical form is defined for the method ofphysical state determination in configured environments. FIG. 13A showsthe generalized method of physical state determination in configuredenvironments using spectral compression. Starting with 1301 at least oneemitter emits wideband energy 1305 into a propagation medium. Theseemissions are intercepted and processed at 1302 by at least oneinterceptor, which produces observables 1306. The processing 1302applies at least one method of spectral compression. Observables 1306from at least one interceptor are processed at 1303 to determine theestimated relative physical state 1307 between at least one emitter andthe interceptor. These estimated relative physical states are reportedat 1304 resulting in a report of physical state 1308 that is externallyconsumed. The reported physical state may also be used to update 1310the system configuration data 1309, providing a means to calibrate andadjust system operation in response to changes in the state of variousinterceptors and emitters. As specified by configuration data, physicalstate 1308 can be reported either relative to a reference point, in theinternally defined reference frame, or in an external reference frame asdetermined by an externally provided transformation matrix.

From this method, all variations may be derived, and thus it serves tofurther explain the essential processes at work in all embodiments ofthe present invention. An important benefit of this generalized methodis that the processing is defined without respect to implementation.Constraints of physical location and communication between processingelements 1302, 1303 and 1304 are purely a function of the logicalarchitecture of the system to which the method is embodied. Differentphysical arrangements of the processing can provide certainoptimizations as required. Processing blocks 1302, 1303 and 1304 aremost often physically arranged to minimize communication bandwidth andreduce power requirements on the location sensor, as discussedpreviously herein.

FIG. 13B illustrates in more detail the intercept and process element1302 of FIG. 13A. Wideband energy emissions 1305 are intercepted at 1311resulting in the intercepted wideband emissions 1314 that are operatedon by some non-linear operation 1312, which produces narrowband data1315 containing the changing physical characteristics needed to performphysical state estimation. Further processing is performed at 1313,which extracts these useful changing physical characteristics. Theseresult in observables 1306 for the interceptor for at least one epoch.The observables may contain at least one or more of the changingphysical characteristics between the interceptor and at least oneemitter. For spectral compression, these are most often represented asfrequency, amplitude and phase for each wideband emission interceptedand for each instance of a non-linear method applied. Each distinctnon-linear operation implementation forms a channel for which multiplewideband interceptions may be observed in 1306. Specific non-linearoperations on the intercepted wideband emissions 1314 in 1312 for theinterceptor may include but are not limited to: squaring where 1314 ismultiplied by itself; delay and multiply where 1314 is multiplied by adelay version of itself and the amount of delay is determined by one ofthe known or suspected physical characteristics of the wideband energyemission (e.g. the chipping rate of the modulating CDMA PRN spreadingfunction); bandwidth synthesis, where 1314 is sampled in two differentbands of a specific bandwidth and frequency offset such that whenmultiplied together they produce a single resultant narrow band data,where the frequency offset, bandwidth are a function of the physicalcharacteristics of the wideband energy emission; differentiation, where1314 is differenced with itself producing the approximate firstderivative; and decimation, where 1314 sample rate is reduced resultingin a narrowband output that is a fraction of the wideband energyemission. For differentiation, additional derivatives can be produced byfurther differencing the previous derivative of 1314. For decimation,the decimated output may utilize aliasing or down conversion andlow-pass filtering to limit the narrowband data to the band of interestthat contains the desired physical characteristics.

FIG. 13C shows one embodiment of the narrowband data processing element1313 in FIG. 13B. Narrowband data 1315 is operated on by a fast Fouriertransform (FFT) resulting in the frequency space transform of 1315(amplitude, frequency, and phase). These data are then processed by apeak detector which preferably extracts the amplitude, frequency andphase for peak values that meet certain requirements as specified by theconfiguration data 1309. Typically peaks are selected that meet certainthreshold value (e.g. 5 amplitude signal to noise ratio) and frequencyrange (e.g. must be between −10 and 50 Hz.). The selected peaks for eachchannel are grouped to form observables 1306, which contains thefrequency, amplitude, and phase values for at least one epoch.

FIG. 13D shows an alternative embodiment of the narrowband dataprocessing element 1313 in FIG. 13B. Narrowband data 1315 is processedby at least one or more phase tracking loops 1322, which are configuredto track signals corresponding to the expected frequencies containedwithin the narrowband data. Each tracking loop 1322 outputs frequency,phase and an estimate of signal to noise ratio, together forming a setof observables 1306 for at least one epoch. Various types of phasetracking loops can be implemented depending on the requirements of theparticular application. Often, the tracking loop will be implementedwith some sort of rate aiding capability enabling a very narrowpost-detection bandwidth that can increase integration time resulting inbetter signal to noise ratio and measurement precision.

FIG. 13E shows yet another alternative embodiment of the narrowband dataprocessing element 1313 in FIG. 13B. Narrowband data 1315 from at leasttwo interceptors are selected in 1331 forming narrowband data 1335 fromthe first interceptor and narrowband data 1336 data from the secondinterceptor. Narrowband data 1336 is delayed in time with respect to1335 by an amount specified by configuration data and/or an amountdetermined the physical states of the emitters, the first interceptorand the second interceptor. The resultant narrowband data is then crosscorrelated producing correlation data 1337, which indicates the maximumand minimum correlation values as a function of time. These data arethen processed by 1334 detecting the maximum correlation peaks, whichresults in extraction of changing physical characteristics between thefirst and second interceptor. 1334 can be implemented in a number ofways but the most common methods are to employ delay locked loops orFFT/correlation peak detection similar to that in FIG. 13C. Observablesproduce in 1334 are typically frequency, phase, and signal to noiseratio.

Hybrid Spectral Compression and Cross Correlation System

An alternative embodiment of the present invention for high-accuracy androbustness is to combine spectral compression with cross-correlation.Spectral compression enables correlation lock without search of thefrequency space for the differential carrier frequency offset since itcan be determined directly using spectral compression observables.Methods and systems for hybrid spectral compression and correlation aredescribed below with reference to FIGS. 14-17.

FIG. 14A illustrates an alternative embodiment of an interceptor usingspectral compression combined with a cross correlation signal processingand a priori configuration data. Hybrid spectral compression andcross-correlation enable direct resolution of the code phase ambiguityby means of a one-time cross-correlation. Configuration data providesthe means to associate cross correlation observables with theobservables produced by spectral compression. This embodiment of thepresent invention is applicable to RF spread spectrum emissions where apseudorandom sequence is used to modulate a signal and data spreadingthe information over larger bandwidths. An example of RF spread spectrumemissions is code division multiple access (CDMA) signals employingbi-phase shift keying (BPSK) or quadrature phase shift keying (QPSK)modulation.

The inherent utility of this embodiment is to provide a simple way todetermine the whole number of code phase chips using cross correlationto resolve the ambiguous phase observables produced by spectralcompression techniques. For example, this technique makes possible amulti-channel receiver that can acquire all the GNSS signal emissions inview using spectral compression and then through a one-time crosscorrelation, resolve the code phase ambiguity, thus enabling thegeneration of traditional code phase observables typical of GNSSreceivers without the requirement of the traditional multichanneltracking loop methods. The intercepted emission 1206 is intercepted byfront-end 1410, which transforms the signal into a baseband regimesuitable for signal processing by spectral compression and other means.For an RF spread spectrum emission, the front end down converts thesignal to baseband or IF, which can be digitally converted using an ADCfor DSP or processed by analog means. In the preferred embodiment of thepresent invention, the front-end down converts the intercepted spreadspectrum emissions to baseband and digitally samples the signals usingcomplex quadrature processing techniques. The baseband spread spectrumsignals 1420 are subsequently processed by Spectral Compressor 1205 andCross Correlator 1411. The Spectral Compressor 1205 use the samespectral compression techniques described for FIG. 12, where one or moremethods of spectral compression utilizing a nonlinear operation areimplemented to produce one or more observables 1421, comprisingamplitude, frequency and phase information of the intercepted spreadspectrum emissions.

Continuing with FIG. 14, these resulting SCP observables 1421 areutilized by a Signal State Estimator (SSE) 1412 to determine theinterceptor's local oscillator state as well as the frequency offsetsfor one or more emitters contained within the intercepted RF signalemissions. Using the Configuration Data 1208, which comprises stateinformation for the reference points producing the RF signal emissionsand an approximate physical state for the interceptor, the SSE uses amodel for the signal that generates a replica that enables thedetermination of physical characteristics, amplitude, frequency, phase,and temporal derivatives for the spread spectrum code chipping clock(code clock). These determined physical characteristics provide themeans to associate the emissions with an emitter by frequency asdescribed previously. For the case where the code clock frequency offsetprovides the means to identify the emission (e.g. GNSS), the carrierfrequency offset can be immediately determined given that the code andcarrier frequencies are typically a fixed ratio that is known a priori.

In the preferred embodiment of the present invention, the SSE producesobservables 1423, which are associated with one or more signal emitters.These observables are then used by Cross Correlator 1411 to construct aspread spectrum code replica clocked at or near the frequency of theintercepted emission of interest. Using this code replica, the CrossCorrelator determines the whole code phase offset of the interceptedemission relative to an internal time epoch. This cross correlation istypically done by accumulating a sufficient number of samples todetermine an unambiguous point of correlation, which can vary dependingupon the type of code sequence used to generate the spread spectrumemission. The whole number of code chips 1422 are passed to the SSE,which combines this information with the ambiguous phase observablesproduced by Spectral Compressor 1205 to produce an unambiguous codephase observable comprising the whole number of code chips andfractional phase. These data are then used to update Observables 1423,which are essentially equivalent to observables defined in FIG. 12 withthe added information of unambiguous code phase.

This alternative embodiment of the present invention requires the use ofconfiguration data to associate the spectral compression observableswith at least one or more spread spectrum emitters. This approach makesit possible to collect unambiguous observables without the need toimplement tracking loops (e.g. a Costas Loop in a GPS receiver). Thisallows the receiver to operate in high dynamic regimes as extensivesearching for the spread spectrum emission in a 2-D space of code andfrequency offset is essentially avoided. The spectral compressionobservables make it possible to determine a precision frequency offsetestimate needed to successfully demodulate a particular spread spectrumemission continuously over time. Further, the cross correlation functionneed only be used once during initial signal acquisition to resolve codephase ambiguity. Once this is determined, only spectral compressionobservables are needed to produce the full set of observables, as anychanges in the whole number of code phase cycles can be readilydetermined by continuous monitoring of the SCP observables.

An alternative embodiment of the present invention is to reduce thefunction of the signal state estimator 1412 to perform only emitteridentification without additional filtering and smoothing ofobservables. In this case, the outputs 1421, 1422 are used directlywithout processing by the SSE.

In GNSS spacecraft navigation applications, where it is desirable to useGNSS signals to track the position and velocity of spacecraft in realtime, the hybrid spectral compression and cross correlator embodiment ofthe present invention has particular utility. In these regimes, thedynamics can cause more than 30 kHz of carrier frequency offset due toDoppler frequency shifts making signal acquisition more complicatedusing traditional code correlation methods. The introduction of thespectral compressor with the cross correlator eliminates the frequencysearch, preferably allowing acquisition of the classic code phaseobservables in four to five seconds from a cold-start condition.Additionally, this alternative embodiment of the present invention canbe readily implemented in a lightweight software defined radio (SDR)form factor making it suitable for operations in multi-use radios.Alternatively, this approach provides significant simplification of thereceiver that enables reduction size, weight and power requirements.

FIG. 14B is yet another alternative embodiment of the present invention,similar to FIG. 14A but with the addition of one or more signalprocessing channels that makes it possible to extract telemetry andcarrier observables from one or more intercepted emissions. Associationof cross-correlation and spectral compression observables areaccomplished by evaluating the carrier phase observables at eachidentified spread spectrum code and carrier frequency offset determinedby the cross correlation and spectral compression observables. Thisembodiment has the added benefit of providing a means to extractconfiguration data directly from the intercepted spread spectrumemissions as well as producing higher resolution carrier signalobservables, which can provide significant value for precisionpositioning and navigation and other science applications includingspace weather (e.g. radio occultation and ionosphere scintillationmeasurements). Within this embodiment, the spectral compressor enablesthe collection of carrier frequency and telemetry observables withoutthe need of conventional tracking loops to determine and maintain codelock. When compared to conventional GNSS implementations, thisalternative embodiment of the present invention makes it possible toproduce the conventional high-quality GNSS observations for one or moreintercepted signal emissions without the equivalent complexity offeringrapid, deterministic signal acquisition.

Spread spectrum emissions 1206 are converted using a front end to abaseband regime suitable for processing by Spectral Compressor 1205 andCross Correlator 1411. The Signal State Estimator 1412 uses theobservables 1421 to determine the approximate code clock frequencyoffset and then configures the Cross Correlator 1411 to search for oneor more correlated emissions. For a particular spread spectrumtransmission system (e.g. GPS, GLONASS, CDMA cellular), there will be aspecific set of spread spectrum sequences that are known a priori. TheSCP observables contain information for one or more emissions that forma two-dimensional search space: a first axis is the quantity M observedcode frequency offsets in SCP observables 1421 and a second axis isquantity N known possible spread spectrum code sequences. This comprisesa search space of M times N combinations.

For example, given five distinct code frequency offsets, M=5, and fivecorrelated emissions, N=5, there are 25 possible combinations to search.In certain cases, there may be one or more correlated emissions for aparticular frequency offset depending upon the measurement resolution ofthe spectral compressor or limited dynamics causing the code clockfrequency to be nearly identical for one or more spread spectrumemissions. In this case, the number of correlated emissions will begreater than the code frequency offsets.

The Cross Correlator 1411 produces observables 1422 comprising theintercepted spread spectrum code sequence identifier and whole codephase. This information is passed to the Signal State Estimator 1412,which then provides updated spreading code observables for allintercepted emissions to one or more Signal Processing Channels 1416 tomeasure the carrier frequency offset. Each Signal Processing Channel1416 is assigned a code frequency offset and one intercepted spreadspectrum sequence. The resulting Carrier Observables 1426 are providedto the Signal State Estimator 1412, which then determines if a match ismade. The processing continues until observables 1421 are matched withcross correlation observables 1422 (comprising whole code phase andspread spectrum sequence identifier). If a match is made, when theobserved carrier frequency offset is determined to be nearly equivalentto a fixed multiple of the code clock frequency contained withinobservables 1421. With the code sequence identifier now associated withthe SCP observables complete spreading code observables 1423 areavailable with whole and fractional code phase, code clock frequency,code clock amplitude and spread spectrum sequence identifier informationfor each intercepted a spread spectrum emission observed by blocks 1205and 1411.

The signal processing channels 1416 are also useful in providing carrierobservables, which may include carrier phase (whole and fractionalparts), carrier frequency offset, carrier amplitude and telemetry data.With the code phase established by the Spectral Compressor 1205 andCross Correlator 1411, additional functions to track changes in codephase such as a Costas tracking loop are not necessary, for example, inthe preferred embodiment of the present invention. In an alternateembodiment of the present invention a Costas tracking loop can beimplemented as part of the Signal Processing Channel 1416 if additionaltracking information is needed or as a crosscheck and validation to theoutput of the Spectral Compressor 1205. Depending on the output datarate and dynamics of the system, this alternate embodiment can haveadditional benefits.

In the preferred embodiment of the present invention, the SignalProcessing Channels 1416 is implemented by first removing the spreadingcode in block 1413, which recovers baseband carrier signal and modulatedtelemetry 1424 with a carrier frequency offset resulting from Dopplershift or interceptor oscillator bias. The carrier frequency offset isthen removed in block 1414, resulting in a narrowband baseband samplestream 1425 comprising carrier phase information, telemetry,transmission path effects and other physical characteristics. This bandlimited raw sample stream is then processed by Data/Carrier Recoveryblock 1415 producing Carrier Observables 1426 as discussed previously.In certain applications, the Raw Sample Stream 1425 is useful to provideadditional information such as ionospheric scintillation. The raw samplestream can be down sampled and stored at a relatively low data rate andlimited to the bandwidth of interest. For example, ionosphericscintillation and radio occultation applications, the bandwidths canvary between 50 Hz and 1 KHz depending on transmission path observablesof interest. Higher bandwidths require higher intercepted emission SNRto produce usable results. Lower bandwidths achieve higher SNRbenefiting from longer signal integration time.

In another alternative embodiment of the present invention, the crosscorrelation, search and match functionality provided by the CrossCorrelator 1411, Signal State Estimator 1412 and Signal ProcessingChannels 1416 can be implemented in a single block or otherdistribution. The particular implementation of these functions, whethercombined or distributed, will be present in this embodiment of a hybridspectral compressor and cross correlator interceptor.

Hybrid Spectral Compression and Cross Correlation Methods

FIGS. 15A, 15B and 15C show the generalized method of processingintercepted wideband emissions using the combined spectral compressionand correlation processing techniques of the present invention. Thismethod is an alternative embodiment that produces Observables 1306 andadditional information providing for the whole code phase and the spreadspectrum sequence/emitter source identification suitable for determiningphysical state. This alternative method starts with the interceptedwideband emissions 1314 as shown in FIG. 13B after the wideband energyemissions have been intercepted, 1311.

FIG. 15A illustrates an alternative method for block 1302 described inFIGS. 13A and 13B. Cross Correlation signal processing methods are addedand integrated with spectral compression observables thus resolving theambiguous code phase. In FIG. 15A, the Intercepted Wideband Emissions1314 are processed by using spectral compression methods at block 1501,which is the combined Non-Linear Operation 1312 and Process NarrowbandData 1313 as shown in FIG. 13B. Block 1501 produces the equivalentobservables 1306 as in FIG. 13B, where the observables comprise multipleamplitude, frequency and phase observables including temporalderivatives for the intercepted emissions of interest. At this point,the method described in FIG. 15A deviates from what is described in FIG.13B, chiefly through the addition of blocks for identifying emissions1503 and Cross Correlation of the wideband emissions 1505.

As discussed previously, these methods apply primarily to RF spreadspectrum emissions, where a pseudorandom sequence is used to modulate asignal containing information content modulated on some carrier. Thepseudorandom sequence is typically a code of known structure that can begenerated both by the emitter and the interceptor. CDMA systems such asGNSS (including but not limited to GPS, GLONASS, Galileo, Compass), WLANWiFi, 3G cellular CDMA and W-CDMA systems are all examples of systemswith wideband emissions that would be suitable for processing using themethods described herein.

Continuing with FIG. 15A, the Observables 1306 provide the criticalfrequency information needed to determine oscillator offsets andpotentially identify the emissions when multiple emissions areintercepted as is the case in GNSS. The method proceeds in processingthe Observables 1306 by determining whether the emissions have beenidentified in block 1502. In the case where emissions have not beenpreviously identified, the method proceeds to identify the emissions inblock 1503. The preferred methods for identifying the emissions areshown in FIG. 13B and FIG. 13C addressing the cases where suitable apriori information is available and unavailable. Depending on theparticular application, either or both of the identification methods maybe used and will be discussed subsequently. The output of the IdentifyEmissions 1503 assigns an identifier to one or intercepted emissionscontained within the observables 1306. This identifier subsequentlyenables additional processing in that it is now possible to determinehow the signal was originally constructed; more specifically, whichspreading code was used to modulate the emission. Proceeding, block 1504determines if the whole spreading code phase has been measured.

Given a spreading code comprising of N code chips, the code phase is ameasurement of the whole and fractional chips within a code at aparticular epoch. For example, consider a pseudorandom noise sequencegenerated using a simple shift register and combinatorial feedback.Configured correctly, this shift register will produce a maximal lengthsequence of N=2^(M)−1 chips, where M is the number of stages in theshift register. A 10 stage shift register can produce a code sequencelength of N=1023 chips. The Observables 1306 provide ambiguousfractional code phase information, the offset within one chip. At block1505, Cross Correlate Intercepted Wideband Emissions determines thewhole number of code chips needed to align the internal code replicawith the intercepted wideband emission 1314 spreading code at aparticular epoch.

Determining the source identifier in 1503 or producing whole code phaseobservables in 1505 can be performed in reverse order or simultaneouslydepending on the particular implementation and source identificationmethods used. In the preferred embodiment of the present invention, bothof these operations are performed nearly simultaneously using the samebuffered data such that only one sample set of wideband energy emissionsis needed to determine both the code phase and its source. This makes itpossible to execute both steps in a minimum amount of time, chieflylimited by processing resources. In the case where source identificationis not accomplished prior to Cross Correlation in 1505, it is possibleto perform cross correlation with an additional step of searching theset of possible codes and comparing the correlation results accordingly.Correlation results meeting certain threshold requirements will indicatethe presence of a source of emission using the particular code sequence.This information can be stored temporarily in a table indicating thepresence of certain source emissions but not yet associated with theobservables in 1306. This table can be then subsequently used in thesource identification method described in FIG. 15C.

The methods for cross correlating the signal with an internal replicabased on the signal source identifier as determined by 1503 can beaccomplished in a variety of ways depending on the particularimplementation. The preferred embodiment of the present invention is tobuffer the intercepted wideband emissions for one or more whole codecycles and perform a convolution of the buffered emissions with theinternal code replica shifting in quarter or half chip steps. The resultis a set of amplitudes of the correlation values for each step withinthe code. A simple search of this result set for the maximal amplitudeindicates the spreading code offset at the epoch where the wide bandenergy emissions were sampled. This technique is readily implemented inmodern digital signal processing systems. Correlating in at least halfchip steps provides the ability to determine the alignment of the wholecode phase boundaries so that it can be combined with the fractionalcode phase as determined by Observables 1306. Thus, it is not requiredthat the correlator produce high resolution fractional code phasemeasurements as the data is readily available by spectral compression.

In the preferred embodiment of the present invention, some calibrationand adjustment will be required to combine these data types accountingfor filtering and latency due to signal processing implementation.Assuming these are dealt with, it is then possible to combine the wholecode phase measurements from the correlator with a fractional code phasemeasurements in 1306. At this point, spreading code observables 1506 areproduced, which can be updated there after without the need foridentifying the emissions or recalculating the whole code phase as theObservables 1306 provide the means to track the evolution of code phaseand the emission over time. In instances where tracking by block 1501 islost for whatever reason, signal identification and cross correlationcan be repeated. Within systems where the source emissions are stableand predictable for some period of time, identification and crosscorrelation for whole code phase may not be required if previously knownand deemed valid. This assumes that local oscillator state is known whenthe intercepted wideband emission is interrupted or lost.

This method as described with reference to FIG. 15A is substantivelydifferent from traditional spread spectrum receiver designs in that theObservables 1306 provide a direct measurement of the spreading codeclocking frequency without the need to search frequency space todetermine the local oscillator. As a result, use of a Costas Loop orsimilar correlation tracking method is not required. In an alternativemethod, both the spectral compression and Costas loop correlationtracking methods can be combined for validation and cross checking assystems resources are available and appropriate.

The choice to use either or both methods depends on the specificapplication. In high dynamic systems, where there is substantial Dopplershift or in systems where the transmission path can adversely affectcarrier phase tracking, preventing stable operation of the Costas Loop(or similar), the hybrid spectral compression correlation method canhave a distinct advantage as it is a relatively straightforward processto track signals in high dynamics and the emission group phase asobserved by the spreading code clock phase will tend to have differentor less impact resulting from challenging transmission paths. Forapplications where the transmission path characteristics are of interest(e.g. ionospheric TEC, or lower atmosphere radio occultationobservations), the hybrid method enables the interceptor to continue tointercept and process wideband emissions where traditional codecorrelation systems may lose code lock due to an inability to maintaincarrier phase tracking.

FIG. 15B shows the method for identifying observables 1306 with thesource emitter when configuration information 1511, describing theapproximate emitter physical state and interceptor physical state, isavailable. This technique works well in situations where configurationinformation exists about the emitter and interceptor physical state andemissions have deterministic frequency offsets relative to a nominalcode clock frequency. Using this a priori information, it is possible toproduce a synthetic spectrum, particularly in the case of the multipleemissions, where the frequency offsets between emissions are a functionof Doppler shift or intentional frequency offsets. Block 1510, MatchFrequency Offsets, matches the synthetic spectrum with the observedfrequency spectrum contained in Observables 1306 to determine the localoscillator offset and associate the frequency of each observed emissionwith a source emitter identifier.

FIG. 15C illustrates an alternative method for identifying Observables1306 with a source emitter when configuration information is notavailable. Using only basic characteristics of the signal structure(e.g. code sequences, code clocking frequency, and modulation scheme),this method de-spreads the intercepted emission and measures thefrequency offset of the recovered carrier, and compares with theexpected frequency offset determined by multiplying the SCP code clockfrequency offset by the carrier/code clock frequency ratio. The codesequence is associated (thus identified) with a SCP frequency observablewhen the expected carrier frequency offset matches the observed carrierfrequency offset. This alternative method of the present invention hasthe benefit of being able to produce useable code clock observableswithout requiring a priori knowledge of the emitter and interceptorphysical states.

More specifically, in this case, the method matches the observedspreading code clock frequency offset contained within the Observables1306 with the observed carrier signal frequency offset given the apriori carrier/code clock frequency ratio information. Since the codesequence/source emitter identifier for the spreading code is not known,one or more codes may need to be tried in despreading the emissionbefore a signal match can be made. The cross correlation method 1505described with reference to FIG. 15A will produce a temporary tablespreading code identifiers and associated code phase offsets thatprovides the needed a priori spreading code configuration information1521 for this method to proceed. Starting with the first entry in thetemporary table, the code sequence for the source emitter is generatedand offset by the specified the code phase as measured by 1505 and 1306observables in FIG. 15A. Using this internal code sequence replica 1520despreads the wideband energy emission resulting in the despread carriersignal and data modulation. Block 1522, implements one or more methodsto detect the carrier signal frequency offset which is a combination ofphysical configuration, Doppler shift, and interceptor oscillator bias.A match occurs when the observed carrier signal frequency offset isnearly equivalent to the observed code clock frequency offset in 1306multiplied by the carrier/code clock frequency ratio 1523, within thelimits of the measurement precision and uncertainty. If a match is notfound, the process continues with the next entry in the table until allpossible spreading codes are evaluated.

This method should always yield a match in that it provides only thelimited set of possible observables 1306 and previously determinedspreading code configurations. If a match is not found, then it islikely that either there was insufficient signal-to-noise ratio to makea positive match or the transmission path is degrading the coherency ofthe recovered carrier. Data modulation of the recovered carrier willhave a broadening effect on the frequency; however, with sufficientintegration time, it can be assumed that the center frequency of theresulting carrier signal is a fairly accurate observation of the carrierfrequency offset, particularly in high dynamic systems where the DopplerShift is a significant characteristic. In the cases where a match is notfound, additional integration time may be required. In the preferredembodiment the present invention the number code sequence chips bufferedwould be increased to provide higher SNR. Preferably, the number ofchips buffered is a multiple of the spreading code sequence length.

In the case where Doppler shift and frequency offsets do not differsubstantially enough to determine unique code phase frequency offsets1306, then the observed code frequency offset in 1306 are assumed to beequivalent for the affected intercepted emissions and the methoddescribed in the previous paragraph associates the same code clockfrequency offset for each source emissions identified within thetemporary table described above. For example, in 3G CDMA systemsmultiple signals may share the same frequency space and have the samecode clock frequency. In this case it is not possible to determineunique observables for each emissions using spectral compressionsObservables 1306 alone. However, the addition of the code correlationmethod makes it possible to use the observed aggregate code clockfrequency to determine the local oscillator offset and then despread theCDMA signals using traditional Costas Loops for code tracking. TheCostas Loop produces the whole and fractional code phase observables.

The detection methods for determining the carrier frequency offset mayinclude FFT and peak detection, low pass filter and threshold detection,or phase locked loop. The particular method used depends upon the typeof dynamics expected for the system. The FFT of the detection method canbe very effective for systems with minimal Carrier Doppler shift, wherethe frequency space is small. The phase lock loop and filter andthreshold detection methods may be a more efficient implementationapproaches when Doppler shifts are large and the frequency space can bemore effectively covered by focusing only on the specific frequencyoffset as predicted by the Observables 1306 multiplied by thecarrier/code clock frequency ratio 1523.

Once the spreading code observables are determined including spreadingcode/emitter source identifier and whole and fractional code phase asdescribed in FIGS. 15A, 15B and 15C, the interceptor can produce carrierobservables as described in FIG. 16. The method despreads theintercepted previously identified spread spectrum emission, removes thecarrier frequency offset as determined by the SCP Observables andapplies one or more carrier data processing methods to produce one ormore observables collectively known as carrier observables.

Starting with the Intercepted Wideband Emissions 1314, using theSpreading Code Configuration 1521 as determined by the cross correlationand source identification methods, the signal is despread in 1601 andcarrier frequency offset removed in block 1602 given Observables 1306and the Carrier/Code Clock Frequency Ratio 1525. With the carrierfrequency offset removed, additional narrowband baseband data processingperformed produces a variety of observables as required by theparticular application. These additional processing options may includeCarrier Phase Tracking 1603, Telemetry Extraction 1604, or archiving ofa band limited sample stream 1605 for subsequent post-processing.Collectively, these baseband data processing options produce a set ofcarrier observables 1505. These observables 1505 can then be used foradditional processing such as physical state estimation.

The band limit applied to the baseband sample stream is dependent uponthe particular application and available archiving storage. Containedwithin the sample stream are additional observables relating totransmission path, source emitter and interceptor physical stateinformation. For example, when applied to GNSS these observables mayprovide information relating to ionosphere TEC, ionosphericscintillation, tropospheric delay and other data used for space weatherremote sensing. In the case of GPS, cutoff frequencies can range between50 Hz and 1 kHz depending upon available signal-to-noise ratio andapplication requirements.

Example Hybrid Spectral Compression and Cross Correlation

FIG. 17 shows an illustrative example of a Hybrid Spectral Compressionand Cross Correlation receiver suitable for use in intercepting GNSSCDMA signals such as the C/A GPS. This alternative embodiment usescomplex quadrature signal processing techniques to produce highresolution code and carrier observables and extraction of telemetry dataencoded within the signal. This example covers the core signal receptionand signal processing functions needed to produce both Code Observables1734 and Carrier Observables 1750. In the case of a C/A GPS receiver,observables 1734 and 1750 can be used as input for a physical stateestimator to produce position, velocity, and time information of thehybrid receiver. This alternative embodiment of the present inventionsuitable for implementation in a SDR. The hybrid receiver exploits therelative simplicity of the spectral compression in combination with thesensitivity of a full CDMA code correlation receiver which isintercepting wideband BPSK spread spectrum signals 1701.

When a code correlation receiver is operated in an environment where thereceiver must be autonomous or functional in situations of virtually noexternal information, the initialization of the receiver is described asbeing in a cold-start condition. In such a condition, an almanac ofpotential emitter signals may be unavailable or aged beyond reliability.In addition, the receiver may have no credible a priori positionalinformation or any detailed knowledge of the dynamics of the receiver orthe state of the internal time and frequency reference of the receiver.In such a circumstance, the spectral compression subsystem provides avaluable contribution as a receiver configuration state-promptingmechanism without the necessity of a feedback mechanism. The spectralcompression observables provides functional robustness against theinability to acquire GNSS signals in cold-start conditions or whenreceiving GNSS signals propagated through turbulent transmission mediaor when the receiver is in unknown dynamic conditions.

Consider the situation of simultaneously intercepting multiple CDMAsignals from the GPS satellites 1701 which have a code chipping rate of1.023 MHz of the GPS C/A channel. The emitted energy is coupled from thefree space by antenna 1702 and input to a low noise amplifier, LNA, 1703down converted to baseband for complex quadrature processing: in-phase(I) 1704 and quadrature (Q) 1708 signal processing paths. FIG. 17 showsboth in-phase and quadrature processing paths; for simplicity, however,only the in-phase elements are discussed given the equivalence of thequadrature elements. The I and Q processing paths are established by thelocal oscillator, LO1, 1705 operating at 1575.42 MHz whose input isprovided by the receiver's internal reference oscillator 1706, operatingat 4.092 MHz. The LO1 frequency is precisely a factor of 385 times thereference oscillator frequency of 1706, which provides the samplingsignal for the analog-to-digital converters, ADC, 1707. The localoscillator, LO1, 1705 frequency is selected so as to place the analogcenter of the wideband signals 1701 at or very near 0 Hz that definesthe baseband region being input to the ADC 1707. The ADC samples with 8to 12 bits of digital of resolution to enable 48 dB to 72 dB of dynamicrange in order to achieve tolerance to in-band interference and tocapture the entire C/A channel modulation width. The output of the ADC1714 is the digitized baseband intercepted spread spectrum BPSKemissions.

These digitized baseband signals are preferably shared with threesubsystems: the spectral compressor (blocks 1713, 1715, 1717, 1720 and1722); the C/A code cross correlator for whole code phase determination(blocks 1733 and 1732); and the BPSK spreading code demodulator/carriersignal processor (blocks 1740, 1742, 1743, 1746, 1747, 1748, and 1749)generate the precision observables of L-band carrier amplitude,frequency and phase.

Using spectral compression, a set of spectral lines is developed fromthe operation of delay and multiply implemented by blocks 1713 and block1715. Block 1713 is a digital delay equivalent to one-half the PRNchipping period of the C/A code of 489 ns or two ADC samples. Thedelayed sample stream is multiplied in 1715 with the original samplestream. The output of 1715 is down converted using multiplier 1717 anddigital local oscillator (DLO 1) 1718 by 1.023 MHz centering theobservables of interest (the recovered code clock) at near 0 Hz. In apreferred SDR implementation, blocks 1717 and 1718 would be implementedusing a CORDIC and phase counter for efficiency. The effect of blocks1713, 1715, 1717, and 1719 is to compress all of the arriving C/Asignals, perhaps a dozen or more, each of 2 MHz bandwidth, into aspectral width determined by the most negative to the most positiveDoppler shift imposed by the receiver to GPS satellite range rate, allof which are centered near 0 Hz. For terrestrial applications thisphysics bandwidth is ±2.7 Hz. For low Earth orbit applications, thephysics bandwidth would typically be less than ±27 Hz for a receiveraboard a satellite.

The baseband signals are detected initially using a FFT 1720 to identifyand measure initial frequency offsets. Following initial observation, aphase lock loop (PLL) 1722 is assigned to each detected spectral linefound in the FFT 1720. Periodically thereafter (e.g. once every 30seconds), an FFT is performed to determine if new signals are availableand to determine frequency offset change rates to improve PLL tracking.The PLL tracking mode allows flexibility where the spectral line isundergoing rapid changes in Doppler frequency for which the FFT modeexperiences straddling between FFT bins with its degraded SNR effects.In high dynamic systems where the Doppler frequency rate is significant,a rate-aided PLL is recommended. Aiding data can be observed by the FFTor calculated using GPS almanac and receiver state information. Theoutput of the FFT and PLL 1721 and 1723 are the amplitude, frequency,and phase observables for each intercepted C/A spread spectrum signal.The phase values are ambiguous at 293 m and can be connected across manyobservations to produce range change observables with sub-meteraccuracy. Precision Doppler observables can then be generated with noambiguity that will allow subsequent positioning, velocity and timingestimation.

This family of spectral lines is specifically the chipping frequencythat is to be applied when generating a code replica in order to acquirea particular GPS satellite by matching a particular PRN sequence andcode chipping rate. By recovering the actual chipping frequency arrivingat the receiver, the effects of the receiver's reference oscillatoroffset and any Doppler shift are explicitly accounted. This criticalinformation of combined oscillator offset and possible receiver Dopplershift has been derived without knowledge of which C/A signals werepresent in the received baseband sample stream 1714 or their codesequences or carrier tracking or telemetry decoding. The internal clockwill in general have an arbitrary starting epoch.

The Signal State Estimator (SSE) 1733 uses the observations 1721, 1723,and 1735 to associate PRN ID and whole code phase observations fromCross Correlator 1732 with the spectral compressor observables. Asdiscussed previously, matching these observables can be accomplished bymultiple methods: 1) GPS almanac and approximate receiver physical stateor 2) matching of code clock and carrier frequency offset observablescontained in 1720 and 1750.

Buffer 1733 stores at least one whole code cycle of the C/A (4092samples), which is then operated by the Cross Correlator 1732 todetermine the whole code phase offset of one or more received C/Asignals. The method of the preferred embodiment is to perform aconvolution of an internal C/A code replica with the buffered receivedsignals. Correlation is considered achieved when a particular maximummeets the correlation criteria (e.g. signal threshold). If a definitivemaximum or minimum is not found for a specific signal additional wholeC/A code cycles may need to be buffered to increase integration time:for example 1 C/A code cycle is 1 msec integration time and 5 C/A codecycles are 5 msec of integration time. Weak GPS signals can requiresignificant integration time greater than 20 msec, which also requiresmanaging telemetry bit shifts if done coherently. The choice of whichcross correlations to perform depends on the search space, which can beas much as 32 different codes for C/A GPS if no GPS almanac is availableand satellite identification was not performed previously using SCPobservables 1721. Given the sampling clock rate of 4.092 MHz, minimumresolution of the correlation is one quarter chip, which is more thansufficient to resolve the ambiguous code phase in observables 1721 and1723.

The spreading code demodulator/carrier signal processor blocks 1740,1742, 1743, 1746, 1747, 1748 and 1749 produce precision carrierobservables 1750. One set of these blocks (comprising both in-phase andquadrature) is required to track each intercepted C/A signal ofinterest. As many as twelve sets of these block is required tocontinuously track all C/A signals in view for terrestrial applications,as many as 18 sets may be required for orbital applications.

To obtain the carrier observables the intercepted signals aredemodulated using a selected C/A code sequence generated by a CodeGenerator 1746 clocked by a Code Clock Generator 1747. The code offsetand code clock frequency offset our determined by the SSE results 1734.The resulting demodulated carrier and telemetry 1741 are down convertedto baseband by removing the carrier Doppler shift also determined by theSSE results 1734. The baseband data is then low pass filtered 1743,limiting the bandwidth to the specific information of interest. In mostcases the bandwidth is limited to 100 Hz or less for extractingtelemetry and tracking the carrier. As discussed previously, however,the bandwidth may be as much as 1 kHz in order to provide additionalinformation for transmission path observables such as ionosphericscintillation. Block 1749 performs data/carrier recovery for bandwidthlimited complex quadrature data 1744.

Direct GNSS Long Code Signal Acquisition

The present invention has application in providing an alternative meansto acquire long code signal acquisition in GNSS. Long codes are oftenused with GNSS systems in that the infrequent repeat interval improvesSNR and eliminates any ambiguity resulting in higher performance. Tounderstand how the present invention accomplishes this, consider the GPSPrecise Positioning Service (PPS) P(Y) channel with an assigned codesegment length of 6.187104×10¹² chips for a seven day period; thepresent invention can achieve code lock without first acquiringcorrelation lock on the C/A channel. Direct acquisition provides thebenefit of allowing acquisition of the P(Y) channel in situations wherethe C/A channel is unavailable or jammed. The essential receiverelements have been discussed previously and the illustrative receiversystem in FIG. 17 can be readily adapted to process P(Y) spread spectrumsignals by increasing the baseband digital sample rate to 40.92 MHz andconfiguring the cross correlator and code generation blocks to generateP(Y) code. In situations where the PPS is encrypted using the Y code,the correlator and code generation blocks will require cryptographickeys to produce the appropriate code values.

Direct long code acquisition of the present invention employs themethods of hybrid spectral compression and cross correlation disclosedabove and adds physical state estimation and successive approximation toproduce time and location information ever increasing measurementprecision and accuracy that ultimately yields correlation of the longcode. For clarity the method is described using the GPS system, but itmay be applied to other GNSS systems as well. The method starts with apriori configuration information including a GPS almanac describing theposition, velocity and oscillator state of the GPS satellites and timeat the intercepting receiver good to then a few seconds of GPS time(wrist watch accuracy). The first step is to acquire the spectralcompression observables for the GPS P(Y) channel. As discussedpreviously, the intercepting receiver is configured to form delay andmultiply tuned to recover the 10.23 MHz chipping rate. Next, therecovered code clock amplitude and phase values for each of the visiblesatellites are identified using the GPS almanac.

The next step is to use code clock frequency observables (containingDoppler frequency information) to determine the approximate position,velocity, epoch time and receiver clock state of the receiver to betterthan 1.5 km and epoch time uncertainty to better than 100 msec.Achieving this measurement accuracy within a particular observationinterval depends upon geometry and the relative dynamics. Forterrestrial applications an observation interval of approximately 90seconds and good PDOP (<3) with a minimum of four different satellitesshould be sufficient to achieve this accuracy. Using the approximatelocation and epoch time, it is possible to localize the P(Y) code searchspace to about 1 million chips, where the uncertainty is due primarilyto the uncertainty in epoch time. Next, starting with the satellite thathas the highest SNR and high elevation angle, the cross correlatorbegins the search for correlation. The search continues until a maximumor minimum are found. The length of the buffer to use in the search isdependent upon the required correlation lock threshold certainty. Longerbuffers will slow the search process but produce more definitiveresults. For maximum search speed, down sampling the buffered samplestream to 10.23 MHz will have the benefit of reducing the actual numberof convolutions to perform. For example, a 1 msec integration time willrequire 10,230 buffered samples to accumulate for each correlation;searching all 1 million possible correlations using a 1 GHz DSP could beaccomplished in less than 15 seconds assuming (1 clock per add). Longeror shorter integration time scales linearly. Once the maximumcorrelation whole chip offset is determined, the higher sample rate datacan be used to determine the fractional chip correlation offset. Thefractional code phase can also be further refined using the spectralcompression observables using the methods discussed previously.

The initial correlation lock determined by the first correlation of thehighest elevation satellite allows for immediate improvement in epochtime certainty by resolving the 0.1 microsecond ambiguity intervalcontained within the previously observed code clock phase data.Worst-case, time estimate improve to an uncertainty of a 10 microsecondsthat drastically reduces the search space for all subsequent satelliteP(Y) channel correlations. Once the epoch time estimate is updated bycombining the initial correlation with spectral compression code clockphase (discussed previously in reference to FIGS. 15A, 15B and 15C), thecross correlation processing proceeds with the next highest SNRsatellite, now with a search space less than 100 chips given time betterthan 10 microseconds and 1.5 km position uncertainty.

Cross correlation of P(Y) signals for each observed satellite continuesuntil the code phase (whole and fractional) observables for three orfour satellites are produced. After the initial correlation of the firstP(Y) signal, the remaining searches typically complete in less than onesecond given a 1 GHz DSP. At this point sufficient P(Y) channelspreading code observables are available to again process with a PSE toproduce an updated estimate for position, velocity and time, at themeasurement accuracy available by the GPS almanac. Next the remainingobserved satellites are cross correlated and tracked if found.

Obtaining higher accuracy requires extraction of the precision ephemeriselements, which are encoded in the L1 band P(Y) channel. This step inthe process can take 30 seconds for the precision satellite ephemerisand 12.5 minutes to extract the entire GPS message. With all signals nowidentified, tracked, and precision ephemeris extracted, the PSE canprovide the full-accuracy of the Precision Positioning Service.Conventional or hybrid P(Y) tracking techniques can be applied at thispoint as discussed previously.

Alternative Spectral Compression Methods

FIG. 18 shows another alternative nonlinear operation for block 1312 inFIG. 13B that applies two or more sequential delay and multiplyoperations 1803 . . . 1804 on a given baseband RF signal 1801, resultingin narrowband data 1802. Each delay and multiply operation may besuitably tuned given choice of delays to target spectral content ofinterest. This technique is useful for processing polyphase signals suchas QPSK, where each step of the cascaded operation reduces the number ofpossible phase states by a factor of two. For example, a QPSK signal hasfour possible phase states. The first step in a two-stage cascaded delayand multiply operation reduces the QPSK signal to a BPSK signal with twopossible phase states. The second step reduces the resulting signal to amono phase state that will produce strong spectral line content for therecovered carrier. While this technique is not useful for recoveringmodulated data content, it provides a useful and effective means forrecovering amplitude, frequency and phase information relating to therelative physics between the signal emitter and interceptor. Certaintypes of squaring can be applied sequentially as well using thepresently described method, where the delay values are all chosen to bezero. For very strong signals, simple squaring may produce usefulobservables but for signals that are weak; specifying delay valuesgreater than zero will have the beneficial effect of randomizing thenoise, producing higher signal-to-noise ratio observables for a givenpost detection bandwidth.

While the preferred embodiment of the invention has been illustrated anddescribed, as noted above, many changes can be made without departingfrom the spirit and scope of the invention. Accordingly, the scope ofthe invention is not limited by the disclosure of the preferredembodiment. Instead, the invention should be determined entirely byreference to the claims that follow.

What is claimed is:
 1. A system for providing physical state informationcomprising: a plurality of ranging signal transmitters (RSTs) that eachemit a spread spectrum radio frequency (RF) emission at a correspondingpredetermined spread spectrum chipping frequency within a transmissionmedium, wherein a given RST is designated as a reference spectralcompressor and translator (SCT); an interceptor that receives the spreadspectrum RF emissions emission propagated through the transmissionmedium from each of the plurality of RSTs: a spectral compressorconfigured to process the intercepted spread spectrum RF emissions usingspectral compression utilizing a non-linear operation to produce at setof observables for the plurality of RSTs suitable for physical stateestimation; wherein the non-linear operation comprises: splitting eachspread spectrum RF emission into an in-phase component and delayedcomponent; mixing each of the in-phase and delayed components to recovera corresponding predetermined spread spectrum chipping rate; wherein aset of reference observables that characterizes a physical state of thereference SCT is received from the reference SCT; a physical stateestimator configured to determine at least one member of the relativephysical state between the interceptor and the plurality of RSTs basedon the non-linear operations and on the set of reference observables. 2.The system for providing physical state information of claim 1, furthercomprising: a reference network processor configured to provide almanacdata corrections for the physical state estimator, wherein the at leastone member of the relative physical state between the interceptor andthe plurality of RSTs is further based on the almanac data corrections.3. The system for providing physical state information of claim 2,wherein the almanac data corrections comprises an estimated state vectorfor each of the plurality of RSTs.
 4. The system for providing physicalstate information of claim 2, wherein the reference network processor isfurther configured to compute a physical state of each of the pluralityof RSTs.
 5. The system for providing physical state information of claim4, wherein the almanac data corrections further comprises coefficientsfor a propagation model that enables the almanac and corrections data tobe employed at a future time.
 6. The system for providing physical stateinformation of claim 4, wherein the physical state estimator is furtherconfigured to estimate a position of each of the plurality of RSTs. 7.The system for providing physical state information of claim 6, whereinthe physical state estimator is further configured to estimate avelocity of each of the plurality of RSTs.
 8. The system for providingphysical state information of claim 1, wherein the at least one memberof the relative physical state between the interceptor and each of theplurality of RSTs includes a position of the interceptor.
 9. The systemfor providing physical state information of claim 1, wherein the atleast one member of the relative physical state between the interceptorand each of the plurality of RSTs includes a clock for a specific time.10. The system for providing physical state information of claim 1,wherein the in-phase component of each split spread spectrum RF emissionis in-phase with a respective spread spectrum RF emission component andthe delayed component is a version of the respective spread spectrum RFemission that is delayed by an interval of about half the correspondingpredetermined spread spectrum chipping frequency.
 11. A method forproviding physical state information comprising: intercepting aplurality of spread spectrum radio frequency (RF) emissions from aplurality of ranging signal transmitters (RSTs) within a transmissionmedium, wherein a given RST is designated as a reference compressor andtranslator (SCT); processing the plurality of emissions using spectralcompression utilizing a non-linear operation to produce a set ofobservables associated the emitter, wherein the non-linear operationcomprises at least two delay and multiply functions; determining atleast one member of a relative physical state between an interceptor andthe plurality of RSTs based on the set of observables and on a referenceset of observables received from the reference SCT.
 12. The method forproviding physical state information of claim 11, further comprising:generating almanac data corrections, wherein the at least one member ofthe relative physical state between the interceptor and the plurality ofRSTs is further based on the almanac data corrections.
 13. The methodfor providing physical state information of claim 12, further comprisingcomputing a physical state of each of the plurality RSTs relative toeach other.
 14. The method for providing physical state information ofclaim 13, wherein the physical state includes a position of each of theplurality of RSTs.
 15. The method for providing physical stateinformation of claim 14, wherein the physical state further includes avelocity of each of the plurality of RSTs.
 16. The method for providingphysical state information of claim 12, wherein the almanac datacorrections comprises an estimated state vector for each of theplurality of RSTs.
 17. The method for providing physical stateinformation of claim 16, wherein the almanac data corrections furthercomprises coefficients for a propagation model that enables the almanacdata corrections to be employed at a future time.
 18. The method forproviding physical state information of claim 11, wherein the at leastone member of the relative physical state between the interceptor andthe plurality of RSTs includes a position of the interceptor.
 19. Themethod for providing physical state information of claim 11, wherein theat least one member of the relative physical state between theinterceptor and the plurality of RSTs includes a clock for a specifictime.
 20. The method for providing physical state information of claim11, wherein the at least two delay and multiply functions comprise:splitting each of the plurality of spread spectrum RF emissions into anin-phase component and a delayed component; and multiplying the in-phaseand delayed components of a respective split spread spectrum RF emissionto recover a spread spectrum chipping rate for a corresponding spreadspectrum RF emission.